Chris, you could possibly find that once you get a few miles under the wheels of your new car the niggles are not as major as you currently suspect. Ian's point about making sure that any raising of the seat includes support for the cushion base is in my opinion very relevant, particularly as you don't want the cushion to submarine if you come to an unplanned sudden stop. Depending on how high you would want to raise the seat, I'd be careful about the design of any significantly raised runner support if this introduces substantially longer fixing bolts. Versus the current design; with longer caphead bolts their heads, nuts, and their mountings will be subject to a bit more tensile stress under high vehicle retardation. However this can be catered for in your design to avoid fixings pulling through etc. Not a long term solution, but to get you motoring, have you looked at Googling "adult car booster cushions?"