The one I've got is fairly thin stainless steel. I have also played on a heavier (thicker) galvanized steel frottoir. The problem with the thinner material is that the part that hooks over the shoulder is too flexible and tends to allow the instrument to work its way loose.
So I'd say: if you're going to all the labor, use a heavier gauge steel. Stainless isn't cheap, but will obviously last longer.
S'b'B
Unless your just into actually doing it yourself, I have both Tee Don's Z Rubboard and a Larry Miller Frottior. Millers is only $70.00 and IMHO better.