I had mine made by ASAP Metal Products in Ventura, they do a lot of fabrication for the Navy and the quality of their work is excellent.
I replaced a set made by one of the aforementioned builders and the differences in build quality were significant, more bends and less welds among other things.
Even the best quality tanks will fail if improperly installed. Make sure the coffin has a drain, allow for air flow around the tank, coat them with coal tar epoxy and absolutely do not foam them in.
If you do a search on the Classic Mako site, there are some excellent threads on the right way to install fuel tanks.