I had a holding tank custom made by local Annapolis plastic fabricators. It is basically a welded "polyethylene" construction. It was certainly cheaper than what the big "rotomolding" tank makers quoted - about $450 for a tank of dimensions somewhat similar to the ones you describe (different shape though, holds about 35 gallons though I try to never get it more than half full).
Just something to consider - another tank option. As an added bonus, I can see how full it is by looking through the wall - it is not entirely clear (thankfully
) but lets some light through just enough to see where the level is. 2 years so far and no smell from the tank. We'll see where that goes.