Lou - I think you'll find that flip-flop soles are not dense enough for outrigger pads. They MAY seem to work, but I think one trip will destroy them. The mud flap or tire approach is a good one, because the material is reinforced internally with cords.
I made mine out of hockey pucks, which I sawed in half to reduce their thickness. They've held up well too, but look like they could be replaced now, after about six? years.
Hi-Lo 1707T - Tire Minder TPMS on Tow Vehicle and Trailer, 300W Solar Battery Charger, Equal-i-zer WDH
2014 F150 Platinum 4x4 3.5L EcoBoost SCrew