A cheap option would be to buy a roll of upholstery foam, wrap it in whatever fabric you want (or a pillow case), and stuff it in the gap. When I brought my basset hound on trips in my WJ, we just left the seats up, stacked luggage on the floor up tot he bottom of the seat cushion and put his bed on top... I'll probably just do the same in my JK.
Tyler R. - Ansonia, CT
Silver '12 JKRU
6-speed, soft top, 2.5" TF Lift, 35" Treadwright Guard Dogs, Smittybilt SRC front bumper, Superwinch TS9500 winch, M.O.R.E. oil pan/trans skid, Skid Row exhaust loop skid, Dominion evap relocation,*Smittybilt Oversized Tiire Carrier and Cibie headlights.