What about a magpul PRS stock?
Check Ehobby, there's like 200 different stock options of an M4 lol

I'd love to get a Magpul stock, but they're stupidly expensive, and it doesn't look like they have much, if any, battery space (not a fan of front end batts)

Plenty of room in the extended buffer tube of the PRS, you could run a 1400mah stick in there easily enough. 1600 if they make one yet. There's even room for a mosfet inside the stock itself.

If you're big on battery life but still want an adjustable stock, you may need a stock pouch for the battery.
Most adjustable stocks don't hold anything big with the exception of crane stocks.
Also the G&P marine battery stock is 1/2" longer than an M16 full stock, but it's poorly designed and breaks easily :/

Hm. Might look into a PRS eventually then. Thanks

EDIT: Was screwing around on that Pimp My Gun thing and saw this stock

http://i.imgur.com/unoXilc.jpg

Had a look around on google and a few airsoft store websites, but can't seem to find one like it. Does anything like that exist for airsoft?

your google fu is weak

http://www.airsoftglobal.com/shop/in...ducts_id=15570

http://airsoft.tiger111hk.com/p20112...duct_info.html

its called a SSR 25 sniper stock