I bought pretty much exactly the flat cage in this comic. It is sold on Amazon.https://www.amazon.com/stainless-G-Strings-Chástí-ty-T-shirt-conduit/dp/B08D8KFB6S/
I am just under 7 inches erect, like 6.75. I can get the cage on including urethral tube, but since I don't have a micropenis it pulls my balls pretty far away from my body and there ends up being like 2 inches of the shaft of my dick exposed. It is very hard to put on since my dick is bulging out of all sides trying to cram it in there. Those 2 posts that insert into the base ring want to pinch the skin bulging. It takes like 5 min to get on.
I do find it incredibly erotic to wear, but I can't wear it for more than like an hour before it becomes too excruciating to stay in it. But I do with I had some evil keyholder to force me to stay in it.
I always boil any urethra play toy before inserting and I use "Surgilube" which is medical grade lube for catheters and has antibacterial properties. It is also on Amazon. Don't fuck around with urethral play. Urinary tract infections are no joke. I have never had one but I am very careful and clean when I do it.
While I find extremely small cages arousing and I fantasize about being locked in one. Realistically a properly fitting cage is the best bet and a standard HT fits me perfect and I can wear it for days with no discomfort. The masochist in me likes to fantasize about discomfort from a small cage, but after a few hours in one the fantasy gives way and reality takes over and I realize the cage needs to come off before I injure myself.
I also have a cage like pic related and I can wear it longer than the flat cage, but not long term.