I figured out the correct dimensions that will work.
1. I don't have a fosgate amp in there and I replaced the rears with Hertz Dieci 6.5 coaxials.
2. I measured the distance from trunk bottom to bracings and it's 19"
3. You're right 15" won't fit but the following will:
The way you do it is to measure front of trunk to back, which is 30". Leave that measuring tape there.
Then opening at front of trunk which is 35"
Then measure from front of trunk to the top of the trunk lip.
Take a second measuring tape, find the number for the depth of the box on the first measuring tape, then measure from there to the top of the tape measure. Subtract that from the trunk height.
MyThat number will be the maximum height that will slide into the trunk at the angle of entry that the first measuring tape is at.
I thought about it and figured out the math. It's triangle geometry.