I need such a cushion to lift me up not slide me forward.
In your case, if you just need to be higher and not closer to the steering wheel, I would suggest to use the seat you have right now as a base, and ask your trimmer to build a new seat thicker (I mean higher) but just in the visible part of the seat, letting the back, the part that slips below the back of the seat, as the original so it still fits below the back and lets the whole properly fixed as original So avoiding any "slippery" extra cushion.
I don't know if I made myself clear…
