Steel or cast iron would be the best choices. Make sure you will be able to get close enough to the work with a tool in a collet with a retracted quill and the riser block installed. You will be increasing the maximum Z axis capacity, but you will also be increasing the minimum Z axis capacity by the thickness of the riser block. Just looking at the photo, you might be OK for most work. Do not factor extending the quill in being able to reach your parts. That reduces rigidity of the setup considerably. That mill is pretty light already and needs all the rigidity it can get, don't compromise it.