Why not make one? It's pretty easy. The simplest would be to sew shock cord around the outside of a square of material that is sized to fit the cover. It can get more complicated (i.e., fit better) but it can be done very simply.
As jimggo says, making one from Sunbrella or equivalent is pretty darn easy. I made one for an old B&G depthfinder display in less than an hour. You sew it inside out, flip it in/out, and voila! Mine fits nice and snug, doesn't need velcro, snaps or shock cord, and has stayed put even in 30-35 knot winds.