if you put some dampening material between the mounting structure of the amplifier, you can mount it to the enclosure. Not too thick though, some foam material would work good, less than .5" thick. You can mount it to the side of the port, where less compression exists. This will help reduce vibrations a little. What I was thinking, was facing the design to the passenger side, then using the floor area to mount the amp using a secondary sheet of wood with dampening material between the wood and the floorboard. You can secure the wires to the board as well. This is just a simple way to have room for a separate "amp rack" per-say, and not have to worry about it vibrating too much. Compression in the vehicle will not be affect much at higher power levels if the design is coupled well with he vehicle compression area. So, phasing will not be much of a concern, just small changes in output at the listening position. These are two options for you. Not the best options, but for a budget you mentioned, it will suffice.
