Very popular amongst Greek ribsters for protection from spray/weather during longer trips. It's called "tenta plefsis"= sailing cover. Often doubles up as a tent for sleeping onboard at night.
Best ones made of "Sunbrella"type weather resistant cloth. Fits over removable arches, and is held by plastic hooks on the tubes as shown on video.
These guys below will make everything on order. For a 6m rib you are looking at around 2 000 E. They are in Greece of course...