I made this to display my Lego Speed Champions and other cars. There are two options: a bracket that supports the chassis pieces that most speed champions are built on, or a bracket that you can snap a real Lego 2x4 plate into and attach anything you want to it, like other cars that do not use those common brackets.
There are these two types in two different sizes in the one file ( For 6x wide cars and 8x wide cars). You can separate them yourself in the slicer and use/copy and paste more of whichever ones you need.
These mounts work on anything up to basically a horizontal surface (yes, anything short of the ceiling. The ones with real Lego plates attached will work at any angle, as your Lego connections will fail before the mount does) YMMV, and depends on the set, I have definitely not tested them all. In fact, the picture you see of my four cars is on a sloped ceiling of over 50 degrees. I added 2 screw holes in case you need to keep them from turning, but I didn't need to use both in most cases.
The Model requires no supports.
There is a hole in the bottom of the plate compatible brackets to allow you to retrieve your 2x4 plate if you are ever done using the bracket for good. You should be able, depending on your printers accuracy, to just snap the 2x4 plates into the brackets and no glue will be required. If its too tight, often its just that the corners are too rounded and you can fix it with a sharp knife, cutting the 4 corners to be a little less rounded. If you cannot, just increase it by 1% in your slicer and use glue if its loose.