It looks like st1264 just has a bunch of Checkmate mags, which aren't exactly earning the best reputation for themselves. You don't want to take your chances with those. The Mec-Gar half baseplate ones are much better (Mec-Gar used to make all of SIG's mags, thanks for the step backwards, Cohen) and should still be easily available, but the problem is that plastic bit. How many broken baseplates... Sigh. (Actually, I have a handful of nickel ones, if anyone was looking...)
What you really NEED is the Mec-Gar all metal ones pinned. They don't have a ridge running up the side, just a diamondey sort of indent at the top. Most of the ones I have are blocked rather than pinned, so they don't look any different than a grown-up's magazine. Unfortunately, I'm 56 bagillion miles west of the middle of nowhere right now, or I'd post some pics.
They fit flush, are metal, and look a lot like the Checkmate mags in st1267's pics, except they have no ridge and actually feed ammunition into the firearm.