How handy are you?
You could get some strap steel from Home Depot, drill out holes for the seat risers, bolt them on. Make sure they extend a couple inches further back than the stock ones. Decide how far back you want your seat, then drill holes for the seat sliders to bolt to that are further back. the front holes would drill through the strap steel and the stock risers, and the rear ones would only go through the steel.
If you were worried about them flexing you could use angle iron instead of strap.