The alum flashing I get around here from Lowes or Home Depot is too thin and soft for a seat built according to the plans.
However you built it, be sure that your seat will not fail. On the single seaters, the controls that are mounted below the seat will be compromised.
We used several sewn layers of heavy fabric in LEU Treehugger, and put a 1/8" stranded control wire in an X shape below the seat as a safety net, just to be sure nothing bad happened from a seat failure. I don't know why we used fabric. I think the woven alum strips are as simple as it can get, and plenty strong.