What CeeBee says is true, but you still have my permission to try it.
Depending on your level of perfectionism, you can probably "massage" steel fenders to match the smaller wheels' curvature better. Playing with different lengths, shapes, and angles as far as the fender braces go will help, too.
FWIW brand new fenders in 20" or 24" flavors cost like $20. 20" and 24" used fenders can be had for prices ranging from "free" to "expensive", depending on variety, condition, and source.