The in-game placeholder extended magazines look pretty poor as they're just stretched versions of the regular magazines. STANAG magazines with higher capacities definitely do exist (and I think Magpul make their own too), the Russians make 40 round mags for the RPK74, but the G36 is an outlier in that as far as I can find, extended magazines are not available (unless you go with a Beta-C drum, which would be awesome on an MG36 please add this for gunner).
When you (NWI) come around to modelling extended magazines for the weapons, you won't be able to find a piece of source material to model for the G36. I have a solution to this issue:
For the same one point cost, instead of having a few extra rounds in the magazine, let's have two mags locked together. Reduce reload time on every other reload by about 65%.
Would also differentiate the M4 and G36 a little further which I feel would be advantageous.