boy, there's a lot of ideas for me here. I was thinking the 4" pvc tucked underneath between cross braces.

(yeah that is showing 6" and 4 to a 2' space, so maybe 5 grey, and 3 cold and 2 hot... that would be about 25 gallons.
now, I can see drain points and pump connection points at the bottoms, but to have the fill points at the tops I need the fittings at the top, so there is another 1/2" of drop below the flooring, at what must be the highest point of the tank. So I'll need like 1" over the "low end."
I do have some regular gas tank straps, but I can't do all of these with those, so plumber's strap and as someone said, doubled up, and cushioned. (and covered against stone impacts, with maybe those thin styrofoam packing sheets and a tarp.
one thing I do know, that the pvc can be drilled and tapped (threaded) and used that way. Just screw fittings right into the pvc.