Chastity device and fitting

I hope this is posted in the correct place.

I would like to know more about how a chastity device should fit, I know its not a case of a correct way but more of a guide, I understand that the male penis is unpredictable in size due to temperature and other distractions.
I have owned a few CB devises and I had a terrible with them, I found them to small and when I did manage to get the device on, it felt like it might snap, they are made well its just they are not for me,
I like the idea of the silicone ones that are available, but I have I have seen mixed reports on them.

Now I have a metal device I purchased this from a well known internet auction site (hangs head in shame) I was very please when it arrived the cage was large and the ring fitted well, its feels strong and that's what I need from a device.
The device was great for a few days then I noticed during cleaning I had some very bad sores on my winky, this is wear the cage is soldered to the entrance ring of the cage, I must be allergic to the welds, so I removed the device and after a few days I healed, not to give up on finally finding a device I have now sprayed my device with heavy duty paint, making certain the welds are covered with a good few coats of paint and now the device is fine.

I am now back in my metal chastity device and it has got me thinking about how a device should really fit, as maybe one day a custom device might be on the cards.

I am happy with the cock ring at the moment, its not to tight so I have a good blood flow to the balls , its just above a snug fit once the cage is on my balls are trapped and it cannot come off.
But should the ring be tight ?

The cage, how should the penis fit in relation to the cage?
When flaccid and at average size should the penis touch the sides of the cage ?
And again with the length of the cage, should the penis touch the bottom when flaccid ?
Or should there be some room?

I know when I was in the CB device my penis was stuffed in, and when I became erect the device was pushed forward and I was left with a very painful lump behind the cock ring where my penis was trying to expand.
In the new device an light erection is possible as I do have some room to grow, but once I fill the cage I cannot do anything

all help and info welcome

AKA Mistress Watchful's Pain gimp