Cockpit cushions work better if they are made in two pieces, makes them much easier to store below when not in use. Sailrite has a bottom material that is full of small breather holes that is very cheap, this material allows any water to drain out of the foam padding in the cushions.
To hold them in place, so they will not slide around have your canvas shop sew a length of 1 inch webbing 1/2 in from the back on the bottom of the cushion the full length and sewed along the inside edge to act as a tab on the bottom to hold a couple round snap buttons. Screw in the male part of this snap into the top of the cockpit seat, back very near the vertical seat back to attach these button to hold them in place, out of the way so that your clothes and shoes will not catch them when the cushions are not in use.