Buy a sheet of exterior 18mm wbp Far Eastern or Malayan plywood from a large timber merchant and some mahogany edging strip and make one. It's only a rectangle after all. Varnish at home to save 5 trips to the boat, take down and fit it while you are off for a weekend.
I made mine in 2 pieces with a piano hinge down the middle so that it folds outwards against itself and doesn't intrude too much into the back deck when it's fully open.
Funnily enough i decided to make one today. I'm self renovating a house and had some 12 mm ply knocking around, so I trimmed it up and put some wooden angle edges on (not sure proper name) I've got to say its starting to look better than I thought it would but I've made it a bit too tight, so tomorrow I'm gonna have to sand the edges to try and bring it to size.