A simple box, open on one side, for a pit garage.
Something like this but a single unit:
https://m.media-amazon.com/images/I/51Vq+F1-3DL._AC_SL1000_.jpg

Most tracks have lots of advertising billboards.
A simple rectangle on 2 legs kinda like this:
https://www.carreraslots.com/Merchant5/graphics/00000001/z5509.jpg

Options for a few colors would be nice. Or, to get really fancy, some GR appropriate product logos :-)