If you are shooting "tactical" style with something like a Harris, then add feet that dig in such as from Hawk Hill. If for something like Fclass, then something solid like thick plywood or steel to establish a flat surface. You will need to experiment with the surface; I found you can see the cross hair move as carpet settles so now I use plywood glued to steel. Some like a slick
Whats the size of your steel plate ?