One heavy piece of ½” plywood. Weight is always a consideration, both as in maximum payload and as in fuel economy. I estimate the weight of most items that I install and keep a total as a reminder. As for the plywood, I’ll need another sheet to finish the job.
To size the plywood, I first align it and extend the lines with a carpenters square. In this case, I first drill a hole with a ¾” forstner bit where the corners of the wheel casing are going to be, followed by the saw cuts, to remove the cut-out. I angled the cuts to follow the sloop of the side of the wheel casing.
The ply will be fastened to the metal floor with a dozen or so metal tap screws.