Alrighty, so you aren't a bot after all. It's just that with how thin the MP5 mag is, and the huge air gaps that would be left if you put one in the mag well, I would not consider that fitting - just coincidence. Not to sound too snarky, but I can fit various parts of my anatomy into the XCR as well too and vice versa - doesn't mean it was meant for me to do that. If a 9mm conversion for the XCR lower was planned, then using the MP5 mag was a bad idea because now there is no room for a proper mag well adapter to cover those gaps. Then again the Tommy gun as an open gap between the magazine and the chamber, and that thing has been shooting for over 80 years, so maybe there is some precedence for that.
Back on topic, I've been having trouble finding sources for materials. That is to say, I've found bar stock of 7075 aluminum from just two sources on the internet and they are going for over $220 for a 14lbs bar of the stuff. Yikes. With all the waste surface area I might get from machining a 2"x6" foot long stock, I'm considering on optimizing my tool paths so I can probably squeeze two lowers out of every bar stock. I hear that magnesium is getting harder to come by, so that's where the higher than expected prices might be coming from. Good time to design while I wait out the shortage, I guess.
I'm not too enthusiastic about making an extended bolt catch for the lower, considering how tricky it can be to precisely machine small metal parts and how I don't have any forging or heat treating tools. At least prices for 4140 steel isn't bad, but I think I'm better off working off of heat-treated stock and then gun bluing the final product; it's the only way I know of to avoid machining off hardened scales. What I really would rather do, though, is print a plastic spacer to go between the regular length bolt catch and the catch foot and attach them together with an inch long socket head cap screw. Maybe not optimal but most of the stresses the catch experiences are at its head, and a longer SHCS should be rigid enough for the shooter's hand to manipulate the foot.