The pts enhanced polymer stock has reasonable space. There's also the vfc qrs. If you want something slimmer you can get a magpul ctr / MOE with an enhanced (bigger) butt pad and bore out the butt pad. Davescustomairsoft do these ready made or there's and airsoft artisan 3d printed butt pad that does the same thing. Lots more choices too depending on what you like the look of.
Nearlty forgot the bcm gunfighter mod 1 also looks like it has some decent space (made by vfc).