I may be biased but I like 32 round mags.
The 20 round mag would be great start for an adapter for the GSG 110 rd drum.
The Lage 40 rounders were too little for me. How did your converted GSG drum function? I remember I was going to try your idea with my Lage kit, but if I remember correctly, there seemed to be some issues with loading the GSG drum. I don’t remember where I read that though. I don’t think I even purchased a drum to play with after reading that. If the GSG drum worked well, it would definitely interest me again. Still waiting for the AM180 drum upper for the M11/9 though. Until then, got lucky and picked up a AM180. Couldn’t be happier with the trigger time of large .22 drums!