This riser has holes for magnets on two sides. I just glued them in so you can hold the riser in place and put it underneath whatever fits on it.
It is stepped in two different increments. 3 mm to the one side and 5 mm to the other side.

I printed four for myself and use them in my laser cutter. You can place them for better repeatability and the increments may help you with different material thicknesses.