When I replaced my black tank in my Outback, I used a toilet flange from Home Depot. It was cemented to the trunk coming up through the floor, then screwed down to the floor. $4 and she was solid.
Cale
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TV: 2015 F-350 DRW, CC, 4x4, with 14k GVWR package, 98 gallon TransferFlow Aux Tank, Timbren Shocks