As with choosing your location for mounting an amp, there are pro's and cons to every situation, owning a 2dr sahara 2012, I am pressed for space. This I decided would my priority when scouting a location for my amp (Alpine MRX-V70).
I came across this post (Thanks hollywoodparke)and decided this would be the ideal location as there is valuable space to be taken advantage of down there.
I worked on hollywoodparke's design and modified it a bit, mainly adding some bends to the bracket that basically lifts the mounting face for the amp up into the seat cavity, allowing more room in the rear footwell for feet and should you get water in the cabin it affords you just a little more room for error, just to be safe I added another inline fuse right there under the seat and mounted it to the bracket for the passenger airbag sensor (passenger seat does not seem to have an actual sensor), I would like to believe that if water where to get into the cabin that 1. the amp if high enough from the floor (about 15 cm), if it isn't 2. the fuse under the seat should be the next line of defense and if not, well that is the sacrifice I am willing to take for a neatly tucked away and out of sight amp that is not taking up valuable storage space.
I used 3mm thick aluminium as the material. I bought a 500mm X 500mm piece of aluminium, took that along wit the CAD file to my local laser-cutting and CNC guy and viola! it took less than than an hour to cut and bend.
NOTE: the bracket in the attached plans was a little too long at the front as it was rubbing on the bar that you would grab and pull up on to adjust for leg room (top of the bracket in the diagram) so I sliced off 50mm, bringing that measurement of 73mm down to 23mm.
NOTE: I recommend bolting the amp to the bracket rather than using self tapping screws, use a bit of lock-tite and cut off any excess bolts protruding from the nuts as these are more that likely to catch on some of the bars running under the seat.
Other than that the bracket fit like a glove on the on the seat rails and with this design the amp does not move with the seat, it only moves slightly when you lift the seat up and forward to get access to the rear seat.
During the install I contemplated sandwiching a 2mm rubber gasket between the amp and the bracket for two reasons, vibration isulation and GRND (-VE) isolation. I decided not to do this and let the bracket act like an additional heatsink. After the install I noticed a significant amount of static build-up, this would discharge if I touched the aluminium bracket, it is not severe at all but kinda got me thinking that I should've done the rubber gasket.
Without further stalling, please see attached dimensions for the bracket in both jpeg format and .dxf in the .zip (draftsight free CAD program).
Apologies for the measurements being in millimeters, here in South Africa we use the metric system (far superior)