Mine has a slat floor which may not provide much support for verticals.
I was thinking of somehow supporting a piece of ply on the main tubes - maybe a semi-rigid pad on each tube to spread the weight and prevent abrasion. Might have to sit in the dunny (Oz for litttle meditation room
) and read sailing mag's for a while & ponder how to do this.
I like your velcro idea to stop slipping side-to-side or for'd/aft.