A tightly woven 1" wide strap has No significant sag when wet .

Cheap common Bungees un hook and get in your wheels and you tear them up,
that is If you don't crash.

I made my own rain covers , the straps that secure them is both side release buckles
and snubbers... a 2x loop of shock cord between the strap and the buckle,
sewing a loop in the strap belt ends,
that you have to pull on tight to close the buckle.