I have been using magic markers to do the numbers for years. It takes about 20 minutes to do and is very easy to touch up. I use a stencil kit to outline the numbers/letters then fill in with a wide tipped permanent marker. They last a good season or more with the dink exposed to sun all summer. Great way to go with no hassle of plastic boards hanging over the sides.

BTW, my dinks are all hypalon, but I doubt it matters what they are made of.