Maybe this is grabbing for straws, but when I needed to go through stainless I had a similar problem. I ended up using a ball stone to wear through first, then continued until enlarged to the desired diameter. This is time consuming, so if you have many holes to make, pack a lunch. You might try a diamond impregnated ball, they seem to cut well. The good news is, this technique will not distort the flat surface, as long as you don't apply excessive pressure. Place a sturdy piece of wood under the material you are working on. When I tried drilling stainless, I got too aggressive with pressure and dimpled the surface. This is an inexpensive option with stones available at HF.