You could try taking the narrowest gap, and widest gap, add them together, divide by two and make shims that thick. Loosen the hood attach bolts just enough to still allow movement with a medium amout of force. Close the hood, place the shims around the perimeter of the hood (a couple per side, and around the headlights and so forth).
CAREFULLY pop the hood just enough for an assistant to get a socket on the bolts.
Never actually tried it, but it seems feasible