Hello; Offhand I can not think of a reason a steel plate would not work. However a few possible issues come to mind. First is that the plate will get moisture under it. That may cause some rust and the rust could stain the counter top. Second is that the plate is heavy and the 20 long could be 200 pounds with all it's stuff. Third is that steel plate is hard to cut. If you do use plate it might help to round the edges and corners a bit. Perhaps a good coat of paint.
I have placed plywood under my tanks often over the years. I use a good grade of plywood of 1/2 to 3/4 thick. I generally cut the plywood the width of a tank and a few inches extra at the long length of the tank. I also have learned to paint the plywood as paint helps to slow the wood from becoming saturated over time.
I am in the process of re-doing the floor of the room where I keep my tanks. I have a 10 gallon tank on a kitchen counter as a tempory place to hold some plants and fish. I have a 20 long but did not want the weight. I have plywood under it. I own the house so will likely be able to work thing out if the tank damages the counter. Probably no issues with your counter but it is alwasy possible.
The big thing is that water makes it's way under tanks and can do things to surfaces. I damaged some furniture from water early on in my fishkeeping.
Good luck
