I can't remember where I saw it, but I think it was a U-tube video. I copied the idea and bought a 6"x6" piece of high density foam rubber. Actually 2 pieces - each 1/2 the width of the mattress. The wife sewed a piece of upholstery fabric and covered both pieces of foam.
The bed is a walkaround - pulling the mattress down somewhat hindered the walking space. So, we only pull it down when getting into bed. No need to extend the plywood. It works quite well. In the morning we take the foam out and push the mattress back up against the wall. The foam is laid on top and its covered quite neatly by the bedspread.
All in all a cheap mod and it satisfied my long legs.