I recently read that a neurologist suggests placing a bar of soap under a fitted sheet as a remedy for RLS.
Her reason for this is that some soaps contain magnesium and magnesium is a natural muscle relaxant.
On the amusing side (i.e. if you find it amusing), I'm not sure what the signficance of the fitted sheet is. Maybe it's something to do with how the magnesium manages to get from the soap onto your legs. In which case it might be more effective for PLMD.
Another reason she gives is that anything is worth trying.
I note her"patient ratings" are at best "fair" only.
On the serious side, I've discovered there are various products advertised as "magnesium" soap.
However, it might be a good idea to wash with it rather than put it under a "fitted" sheet.