update 20200105 - Added v2 stl from onshape document. Uses internal rail edges to keep wheels from sliding sideways.
Just a simple display ramp. There are two more variations on onshape document, but this one is most simple. This on needs to be printed standing on rear wall. Can be printed without support.
Other one uses cut-out plastic from car packaging (like 4x4mm square) glued into ramp slots, as wheel stoppers. And the last one (ramp) is printable on side, as mirror parts, connected with axles/wires/pins/clips; with wire holes adjusted.
It's so thin, that it will end up as 100% infill anyway. All angles are around 65 deg, so it should print OK with good cooling.