[Killer Bee]

in your position, I would get a pair of full range marine/boat speakers with mounting brackets, bolt on some studded magnets and stick them to the floor board under the seat or in the corners behind the seat.. run the speaker wires under the sill plate and tuck under whatever mat or carpet you have.. pad the magnets if paint finish is high quality under there or even hang them from seat frame with plumbers tape or small custom standoffs you could bend up..

good luck!