Adjustable Cage?

It looks horribly uncomfortable, and likely be far more uncomfortable than a steel cage.
Fake leather can have edges that cut like paper, and all those edges, seams and fittings will just add to the discomfort. Everywhere where the leather slides on a ring is a potential skin pinch point.

I'm not even sure how that thing is supposed to stay on.
Are those two straps supposed to go around each testicle?
You'd need a very loose scrotum to even be able to put it on, and then, what will hold it on/up? That soft material design doesn't seem to offer much resistance against pull out, you'd probably just as likely fall out of it so I wouldn't even call it a "chastity device", it's more in the realm of a "posing pouch".

 

How about something like this?
https://a.co/d/b359TJj

It's a basic inexpensive device that comes with multiple ring sizes and the locking mechanism would allow you to use a zip tie or the serialized plastic locks which would give the newcomer to chastity a sense of willingness to try without giving up too much control and trust. Then over time you could work towards a real lock and smaller more secure cages.

Then once you've built a little trust you could step up to an inexpensive internal lock. I've had good luck with this brand.
https://a.co/d/b359TJj

I agree with @bondinchas the leather chastity can be difficult to work with from a fit and feel perspective.

Good luck!

 

I have no idea what that leather/steel contraption may be but it's, at best, good for a play session. If you're looking for longer term chastity then you need an actual chastity device. The unfortunately reality is that most guys try a number of devices before they find one that's really comfortable. On the bright side, the plethora of inexpensive Chinese knock-off devices make for a relatively small outlay.

My recommendation is for a Kink3D knock-off which can be found on eBay from a U.S. based seller for $20 or less. It's an open design which helps with cleaning and they come with 4 base ring which should help eliminate one measurement. Beyond that's it's length, which should be of a normally flaccid penis. Then subtract 6mm to 12mm because smaller is better when it comes to overnight erections and most guys are overly optimistic about their size. ;-)

 

You could achieve that by using a device with a lock, and instead of locking it, use a clip, leave the key in the padlock, or leave it close by in plain sight on a table or shelf. You could even put it on a string around the captive's neck - so close, and yet still so far.

 

I'm not sure about a bondage-contraption that would work to experience chastity without using an actual device. Before I found 'mainstream' devices or specific to chastity 'contraptions' I explored the hardware store looking for 'things' that would reduce stimulation or restrict how much of an erection I could get... I honestly did not know much about chastity, just that in bondage scenes or self-bondage play, it was fairly easy to get off while bound but with the right padding or forced angle/position of my bits it was more difficult... Eventually, I discovered that what I was looking for was a chastity device and with very few of those mainstream devices out there at the time, it wasn't hard to just pick something that I figured would fit, with some options like a range or base-rings and ring-to-cage gaps and just go for it. This was before you could find just about anything (specifically a 'test' model) in the world on sites like Amazon for a fraction of what would require saving up for.
Sometimes, I encourage 'go big or go home' or 'sink or swim' when it comes to getting into chastity... like, go through the paces of getting accurate measurements and find a device that would at least appear to meet those measurements and with the 'you get what you pay for' theory in mind, spend a bit more than you would on something cheap with the hopes that it would be a perfect fit, by 'just going for it'...
BUT, for your inquiry... back to where I started out... my first 'device' was the CB6000s with all the spacers and different size rings. The dinky lock that comes with it is disposable, the plastic 'seals' are a fun way to get used to being locked without the fear of having to break out of the cage... Unfortunately, every knockoff version of the CBx devices is, in my experience, unwearable for any 'real' length of time... The rings' construction is not very comfortable at all, BUT can be a great sizing tool. With the various ring sizes and gap pins/spacers you could call this about the most 'adjustable' cage out there... Unfortunately, not for girth, but at least for all the other parts....
In addition to this... I rarely point to the silicon versions of any device out there, but, to give someone a feeling of their bits being restrained without it being 'too real'... even with a lock, these can be pulled off with very little effort, yet, can feel like actual chastity for a new-comer...
Adjustable girth, though... not sure you're going to find anything out there for this part....

 

I would probably suggest one of those HT knock-off cages on ebay, they usually come with 4 different rings so you can find the size that works best, and they're usually under $20, so not a big financial commitment. Just chose a cage size that you think will work.

Be aware though, that the cheap knock-off cages haven't been hand smoothed like the expensive ones have, so it's a good idea to smooth out the seams with sandpaper, and the edges of the docking station can be sharp, so smooth those out too.

Here's a listing for $16.99 that gives you a choice of sizes and colors, and shipping should be quick from NJ to PA.
https://www.ebay.com/itm/304606442918

 

I have long thought there must be a way to make an inexpensive adjustable device to assist proper sizing, I have a couple ideas on mechanisms, but lack the ambition and tools to advance the ideas. It would have been very helpful to me when I was exploring sizing before dropping $400 on a custom device and then needing expensive and time consuming modifications to have access a device to test the feel of different size cages.

I don't think that leather thing you show as an example isn't really worth buying, it won't fit anyone well and doesn't really adjust much. Also, without any graduations or fine adjustments, it wouldn't serve the sizing purpose well either.