This is a little off topic but when reading about this topic I remembered that back sometime in 82 or so, when building a house with/for my uncle,
We had saved up all the ends cut off the 2x4s used during construction. We took 2 3/4 inch sheets of plywood screwed together. Then nailed the 2x4s on (stacked like bricks)around the perimeter, with a divided compartment in the middle. It must have been six feet or so high. we coated the inside, and outside with black roofing tar. The backhoe set it into the hole dug for it, and its been in operation ever since. Its served a familly of 6 for 25 or so years. Total cost a couple sheets of plywood and a pail or two of tar.
I dont think such a system would be permitted now, and perhaps it wasnt even permitted back then but it sure was cost effective.
If its permitted in your area, its one more option to consider.

John