That's interesting. I don't have a problem between my 9mm and .22 slide. I guess the leather gives enough for the kadet to fit.
I don't know of any holster that is made exactly for the kadet adapter. I would not buy a kydex holster for the kadet unless I could check it's fit first.
I would check local gun stores for Uncle Mike's holsters. Case the gun, show the owner it's clear, and then check them out. I bought a nylon holster with a clip attachment and mag pouch on the front. The size was actually smaller than recommended. I prefer them tight because they loosen up after use.