I don't think I understand what you are asking...
Would you not just put the box down on the surface...
How big in your truck is that surface?
//2006 Jeep GC - 20" Boss 335's - Custom 2-tone leather red/silver - custom projectors - tint 30frt/dbl10rear//
HU: Pioneer 6800UB
Comp. Amp: Alpine MRP 240
Sub Amp: Rockford Power T1000bd
Subs: 2 15" AudioQue SDC 2.5 15's in 9.25cuft @ 34hz
2 - Exide Drycell Batt
If you want a tight fit then built it thats shape then lay a flat piece over the top of the basicly making it a double bottom. flat for the inside of the box and curved to hug the floor. problem solved.