cut a base like in diagram.
then it stands on little wooden or plastic dowels tall enough to raise it off the full size base. dowels go right behind tires where they are hidden.

alternate method.

wood main base

long narrow wood block that is screwed and glued to it and painted black.

then the contoured base in diagram is screwed, centered on that narrow wood block.

narrow wood block would be as tall as you want car to sit off the main base.