Seeking small metal device with external lock

Bon4 Micro maybe? Although not sure if that qualifies as not super expensive.

 

The dimension you're looking for is what's called "gap" and it's almost never part of the information provided by the Chinese manufacturers. I'd be willing to bet that Koala Swim is just reselling Chinese "handmade" devices. Finding a device like the one from Koala is going to be tricky, but you might want to search the offerings at Ali Express and DH Gate. I might also suggest that super short devices generally are not comfortable for along term wear unless you are actually super short. The closed front will also require you to sit to urinate and will likely require regular cleaning because you won't be able to get that last drop of urine.

With that said, the gap of those single post devices can be adjusted with a little skill and tools. Basically, you carefully bend the post to increase the gap.

The Bon4 Micro may be a good fit choice for you, even if it is a little expensive. It's comes with several different sized based rings and the gap can be adjusted using the provided spacers. Being an open design, hygiene will be easier as well. One of our members @OscartheTurtle has the Bon4 Micro and says he likes it. He has a thread in the device review section or I'm sure he would be happy to answer any questions you may have if you contacted him directly.

If you would be interested in a plastic device, I might suggest checking out the Cherry Keeper designed by our own @JosieLynn Jewell. They are well made and an excellent value. She offers a variety of devices, including some super short models. I believe that some can accept a padlock. I'm sure she would be happy to answer any questions you have.

 

I have the same situation - large balls that cannot be squeezed through. I can only wear a hinged ring and the Wife chooses them (she periodically purchases smaller ones). She got me a new one this week in fact, from Koalaswim - it is the "OMG Micro 2" with a 1.5 ring and a 1 inch spiked anti release ring that sets between the base ring and the cage. She rigged me yesterday afternoon and I'm just completing my first 24 hr. period in the cage. Though I've been permanently locked for many years in various cages, this one will require getting used to. Good Luck!