At the time the picture was created, there was ongoing discussion about using ram air to inflate the boattail. So that's what the scallops were about. Today I'd use fiberglass rods in sleeves and/or a 12v fan, as aerohead did on his VW bus. More like an air mattress than a beach ball.
The pivoting floor/diverter would be rigid. When it is pulled most of the way down, the rods are inserted and it is tugged down until it latches with the fabric skin tensed. Inflated tubes could work on a push button on the dash.
Here's an earlier example with the tubes running crossways. I drew this around 1970, a variable-wheelbase reverse trike:
I think it was inspired by the ice sled in Barbrella.