I had another look this morning and cut away one of the ridges on the floor pan. I did not have a dremmel so did it the manual way with a 360 degree rotating mini hacksaw blade.
I drilled a small hole carefully first to see what was under neath and was just metal shielding above the exhaust. I lined up the amp and drew a line slightly bigger around it and cut away. Originally I was going to glue on a persepex sheet over the open hole but again I would be back to square one due to the amp being raised too much. I ended up using roof flashing tape, heating it up with a hair dryer to ensure it bonded to the plastic.
next weekend I will wire it up as i dont have any power cable spare.