Okay... so 3/8" plate for the main tank and 3/16" plate for the freeze tanks? Do you want me to refine the design I made to include the studs and wing nuts along with proper plate thickness? I have already added the fill port and inlet/outlet ports to my drawing.

First thing I would do is practice on some scrap pieces the same thickness that you will be putting together. Make several beads about one or two inches long to get the feel of it. After you've done about a dozen or so you should get the feel of it.