Yep, Gary has a few different ideas on DIY tank designs on his site thankfully. However, I believe his is the cheapest and easiest to build and insulate. A 120 gallon tank retail for $1000+ and I'd need a few of them! My current cost estimate for the tank and liner (no plumbing) is about $250 in materials.
I agree, a square tank would be more thermally efficient. The currently modeled tank can hold about 650 gallons of water. Since I only need 400 I will be shortening it up a bit length wise. However, I still have a rectangular footprint to put the tank in, so that will also be determining the shape of it to some extent.
The tank (and
the rack) is modeled in SolidWorks and analysed with CosmosWorks.