Fairy Liquid and a sponge would work, but if I was you Id just cut some bike tyre to length and screw it on. Drumroll ran at two events at the same weight; one with 100mm blue wheels normally, and one with bike tyre screwed on. The improved grip from the bike tyre enabled me to push other bots about much more than the wheels without tyre, and that was with my robot only weighing 7.5kg.

Whichever method seems easiest and most practical to you though