RC cars have a pliable but tough, clear, adhesive backed material available that is used to protect the bottom of the car/truck, etc. You can cut pieces out of the sheet supplied to conform to the bottom of the wingtip and the self adhesive sticks well to monokote, etc.. It takes the abuse on paved fields and protects the wing. Check in the RC car section of your local Hobby shop.

Edit for spelling, duh.