9mm plastic (not sure if its actually kydex or not) lined Pantac/G&P pouches fit KWA USP mags. I would guesstimate that the MK23 mags aren't much bigger... The G&P pouches fit a little looser than the Pantac ones, but they're still quite tight.
Hmm, doesn't look like Ehobby carries the Pantac ones. Pretty sure Flyye makes them, but I don't see them there, either.
Flyye/Pantac .45 plastic lined pouches fit SINGLE STACK mags, or at least, mags that are modeled after single stack .45's (WE/TM/WA/etc 1911's and such).
Can't say about the 5 mag pouch though.
I've got a whole bunch of these (more than I care to count): http://shop.ehobbyasia.com/combat-ge...ote-brown.html
and I know
they do fit MK23 mags.
Edit: G&P calls their hard pouch lining "FB", FYI.