We have been very happy for the last 10-years with a stitch-and-glue
holding tank of 1/2-inch plywood. Coated inside and out with two coats of
epoxy, it now looks the same as when first constructed. It even has a six imch see-thru port in the top for inspection and maintenance shoud maintenance be needed. There are no odors and no leaks. They can easily be built to custom fit the alloted space and while ours is only thirteen gallons the estimated cost was about $20 including the bronze fittings. I would foresee no problem with doubling or trippling the capacity. Just add baffels to break up the free-surface area.