Find or construct some kind of guideway, rail, track, or the like.
A toy racecar set, for example, can make a great guideway.
You can also use a piece of straight or gently curved wood, or even the edge of a desk.
If you want to curve the spacetime continuum, try "wiggleboard" or a piece of thin wood that can be gently bent.
Make splines out of metal or plastic, if you like