I have a really good idea for you, well i think its a good idea at least lol.
If i was you I would build a box with 2 feet on either side, the exact height as the hump. that way you can have space for amps and wires and you will have good cooling as well, well maybe not great cooling but better then what you mentioned before with the amps in the spare tire well. You will lose a some cubic inches, maybe a foot cubed not having the box with a channel down the middle to use all the space that you have, but with you running 4 8s or 2 10s you still should have plenty of space.
good luck and keep us posted.