I use 1/8 plate. As 1/4 plate is entirely too think and hard to work with. My patterns are made with just card board. Then I trace it out the the steel and cut it out.. I mostly leave a 1/4 to 1/8 gap in the bottom so i weld the plate to the frame and keep it all in the lip.

I use I will verify but from memory 5.5 inch 1/8 bar stock, 3.5 inch 1/8 bar stock and 2.5 bar stock steel. the 2.5 is my bottom of the frame and the other two sizes are my sides.. Middle section is 5 inch.

when an area is completely rotted. I cut it out and plate the inside first. Once that is completed. I will put a plate on the outside was well and weld it all together. This is very common in the Arch areas as they mostly rot out first. By sealing this all up. Not only we get a super strong frame. We get rid of all the holes in the frame. So the paint and fluid film cant go anywhere and just protect the steel. Should make the frames last almost forever. Regardless of the areas they are in. :-]