Yeah I've not fitted it just yet since I'm waiting for a set of Crye side rails to come in but I've actually already fitted an Ops-Core X-Nape to the TMC 'frame, went on no worries. Had to use the original chinese screws and the specific nuts that Ops-Core supplied to get the best setup but that wasn't a problem. Only bought a new one because the one I picked up previously was for left-handed shooters ('Left Eye Dominant' version) which meant the buckle was on the right side and sat exactly at the point where your cheek should meet the stock, kind of annoying. Alright if you use the old 'tracer effect', purely instinctive shoot or already wear a mesh mask I'd imagine but I like to be able to get my face against the gun.
Honestly, I wouldn't recommend Eagle even if you did have the cash, their designs are from the tactical stone ages. Compared to Blue Force Gear for example, who make (in my book) basically the best and most cutting edge line of pouches out there, they really are way behind the times. They might bring out a new line at some point but they seem to be selling so much to the american military that they don't care about civilian sales, which isn't uncommon with gear/gun makers over there. I'll be sure to keep my eyes open for a single-mag only lidded pouch though, as you say they are pretty rare but I definitely get where you're coming from.
The EBBr PMAGs are built incredibly well, frankly OTT for an airsoft mag (which I'm a fan of personally). Nice capacity increase vs the TM 82 rounders and they're the same height as a RS mag which makes a nice change from all the stubby AEG mags out there. Fire-Support had a big stack of the things when I placed my order so you should be alright to leave it for a little bit.