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Always advocating for Cherry Keeper found here for less expensive customs.
And what seems to be coming around as the gold standard is /u/kink3D . He's a redditor and is very responsive too. https://www.kink3d.com/ but they aren't cheap.
If you aren't looking to shell out a lot yet, have you seen this one? https://www.amazon.com/dp/B0851GJTLT/ref=cm_sw_r_apa_i_7EsyFbBWB26KK
It's literally the same one as lockthecock.com . Actually I'm pretty convinced lock the cock just sells the same Chinese knockoffs/mass produced cages that Amazon and eBay do but for a higher markup.
If you have tried that and it actually doesn't work, go with cherry keeper of kink3D. Oh, and if you like metal, though I find it heavy for casual wear, there is maturemetal.com they make very good, but even more expensive, custom metal cages. People swear by them though so that's a plus.
Greymond Male Chastity Cage, Stainless Steel Hallow Chastity Device with 50mm Rings https://www.amazon.com/dp/B0851GJTLT/ref=cm_sw_r_cp_api_glt_fabc_ERMFWFF5WKA514ED6HBM?_encoding=UTF8&psc=1
You're 5-6" flaccid? Yeah... They don't mass produce cages that size, but they do get kind of close.
So this one https://lockthecock.com/collections/metal-male-chastity-devices/products/silver-metal-peniscage
Is the biggest mass produced one you're going to get. I think it's the same as here
But I'm not positive. The first one might be longer, but it could be that they are measured differently.
If this is what you have already bought, then the answer is you're going to need a custom cage. Since you're starting out I'd recommend Cherry Keeper chastity because it's a cheaper custom cage.
Just to make sure the metal one won't work, how thick are you flaccid?
Male chastity device. effective unless turned upside down.