A plastic RV/Boat battery box in the bed, or a fabricated metal box (some offroad places may have this) bolted to the frame might do the trick... If you don't mind me asking, what is your reason for needing an aux battery? Other suggestions or options might come along if the reason is known.
MODS LIST! -- 20% Tint, Curt Rec. Hitch, Slimline Toolbox, Huskyliner mats, Huskyliner Mud guards, 18" Gear Wheels, 33" Hankook ATM Tires, Bilstein 5100's at 1.4", Ported Foxbox w/2 10" Polk Subs & JL 500W Amp, Mufflex 24" Exhaust, Tow Hooks, Dominator II Step Bars, GatorHyde Spray-In Bedliner