[Snowtractor]

...used to work at a rock and roll rotational plastics molding company. We made linear low density polyethylene containers in all sizes. The septic tank was rectangular 2 feet high by 6 wide by 14ft long. With the top cut off it would take a jeep frame no prob. Also the 1200 gallon ( as I recall ) was circular about 5 feet at the shoulder and 8 feet in diameter with a flat bottom. This would take a cab easy. They were about 1000 cdn. We did have the odd piece that didn't turn out or the wall thickness was too small that you could buy for peanuts.
Alternatively a wood crib of appropriate size with a vinyl liner should work. Find someone getting rid of one of those above ground pools and offer to dispose of it for them . Even new they are about 300 bucks for the the small version. Buy it and you have one large caustic tank...just keep the kids out of it.
Sean