King arms do 2 sets of 5 midcaps, 95 and 110 rnd. Regarded pretty highly, but I'm unsure if they work 100% with that model.
Classic army also do some midcaps. 70 and 120rnd. Regarded highly by the CA users but again unsure how well they fit the JG.
LCT also do a 110rns mid cap, but these are more specific to the gun than the others. Chances are these will not work. And even with an LCT G3 they require some work to feed perfectly.
Outside of that I can't help you a lot. Most of the G3 users I've seen use the Classic Army G3. The JG users know from the get go that mags will wobble and the plastic will creak. It's well documented, A bit of reading would have shown this, and is one of the reasons I tried to steer you to the CA gun in the thread you posted a few weeks ago.