Anyway, you've probably seen the Tamiya driver cockpit sets. I've been using them in many of my on-road cars -- when I can find them.

They can be a little hard to get sometimes so I'll cut and fit a piece of black poster board to function as a placeholder. I just use masking tape to hold the poster board in place and it seems to work well enough to darken the driver's compartment, hide the chassis, wiring, etc. - at least until I can find a driver cockpit set anyway.