Yes, you need 1/4 and 1/2" plate. Not much 1/2".
I had 1/2 x 6" CR flat bar in the shop already. You need 2 @ 5" x 6", 1 @ 5" x 9" and 1 @ 2.5" x 9" -- roughly 1 square foot of material I think.
You need more 1/4" plate, about 6 or 7 square feet (not allowing for cutting/waste). You certainly don't need a full 4 x 8 sheet. I bought a 10" wide x 20' long (cut in half for handling) piece of flat bar. That was convenient to work with as none of the pieces are more than 9.25" wide. I plasma cut lengths off of the flat bar, then laid out accurate cut lines and sliced it up on a bandsaw.
Somewhere I posted my shopping list that I used to price out metal.