FYI, I came across this amp on the Crutchfield site. It's a Sony four channel with dimensions small enough to easily fit in the gas tank area between the bulkhead and the filler pipe assembly. I am always on the look out for something like this.
It has the same dimensions as my four year old 4-channel Hifonics.

No need to build a shelf and install a more powerful one, unless you really need the power!


Sony XM-4S
Slim Series 4-channel car amplifier
50 watts RMS x 4
Item #158XM4S

In Stock

$249.99


Specs:

4-channel car amplifier

 

It may also fit under the driver's seat since it is the shortest amp I have come across.

It is a class D amp and has some of the same dimensions of a two channel amp.

 

Measured the open space in the gas tank area yesterday. This is the rectangular space to the left of the gas tank hump, between the rubber covered gas lines and the bulkhead. The space runs 11-inches long, and about 9-inches wide. The space is slightly irregular because of the shape of the bulkhead bracing and the lines angling slightly inward.

You can always fashion a shelf, but matching the amp to the space worked for me.

The new Sony will easily fit. I think that the Alpine PDX amps may also fit in 4-channel configuration.

Some subwoofer amps and quite a few 2 channel amps will probably fit in this area too.

I found out by accident when I laid in my compact Hifonics 4-channels (8-1/2" x 7 3/4" x 2") in the space, but I never actually measured this space before today. I so no other post with this information either.

 

The alpine pdx will fit above the gas tank just fine

i've seen this done a few different times. it's the amp i'm planning to purchase when I have enough extra cash