I built my own for under the rear of the truck between the frame rails of my '71 fj40. FYI I used a tank sealer from http://www.eastwoodco.com/ (then just search for tank sealer) I really like it out of the passenger compartment.
I used 11 gauge mild steel and a wire feed welder added some baffles inside to prevent sloshing. as far as welding goes make several passes to make sure you get all the large holes filled, and then the sealer will do the rest.
Did you create a steel box with a filler neck? Do you have this on your "purple rig" now? How long have you had it? What did you use to cut the pieces? Was this someone elses design? Blah, Blah, Blah.........
I am VERY interested in getting rid of the underseat tank in mine. Any information is greatly appreciated!