I wish i had a pic to show you what I did. I cut a board to have rounded ends, and just the right size to fit down in the spare tire. So the board sits inside the bottom of the spare tire, amp is mounted to the board. If i ever need the tire, i can just pull out the board. The amp has a couple inches of clearence, so it doesnt get too hot.
The amp is never in the way, and you never see it just looking in.