I'd suggest to visit a junk yard and look under some hoods for a typical steel battery tray. I have aluminum and steel plate, angle iron and other, also have a welder. I wouldn't start to make one before shopping the recycle places because the labor and time are too much for the use and longevity required. I have had steel battery frames and cases that I've rehabbed from acid attack. They will last a long time.
If you need something custom, drop me a PM. I am getting busy right now but may have some time to make one (if you can be patient).