So, last night I remembered an idea I had to make a non-orientable track such as a Klein Bottle immersion or a Möbius strip (side note: have SRB2Kart made a Mobius track yet? They should!) and discovered this shape: the umbilic torus.
Now you can race on one!
What is an umbilic torus?
Basically, it's a 3D shape which has one edge and one face. The wikipedia article explains more.
I made a quick-and-dirty version of one by making a deltoid:
Then making it follow a circle path while tilting, then mending the edges:
I could have done a better job if I planned it out properly, so I might end up doing that another day. Also, checklines don't work yet and I'm not sure if they ever will be 100% accurate, but I'll fix the laps at least.
But for now, you can race on it!
Download: https://online.supertuxkart.net/dl/1652 ... 327c00.zip
Or in the add-ons repo.