If you have an old pair of tennis shoes/sneakers you can cut off most of the top (leave enough to use to stitch top to sole) and then use any number of methods to attach the crochet sock/slipper to the sole.
Possible more than you want to spend, but I have always glued in a Dr School type shoe line to give a smooth base. But I tend to make things to last so don't mind spending a little extra.
Long ago I did a pair that attached the crochet slipper using a hand picked zipper. This allowed me to remove the sock portion for washing
Another time I modified a pair of sandles by attach