The size is the only limiting factor. Get the dimensions of a fairly standard 2S pack like the Orion 4800. Then look for a 3S pack with similar dimensions. I've seen some 4000mah packs without a case that should fit fine. Venom also has a pack in a plastic case that looks like it should fit.
Tamiya TRF417X (Lion's Automotive GTR)
Tamiya F104W MP6/4 converted to F104X1 (Renault R30)