My 96 18C came with a factory transport cover. It was a heavy vinyl with a felt liner and had a draw string on it. It fit like a glove. I only used it on long road trips. I have never seen another on a donzi. To have one like that made would most likely cost around $600. For a 22, probably closer to $800.
On the other hand, if you are just looking for a cover to trailer with, that is affordable, you will have to go with an aftermarket, pre-designed cover. The fit will not be as good but you really don't have too many choices. If I remember correctly, West Marine had a selection for many different styles of boats. You may want to check them out and send a few emails. They may have a pattern for your boat.
Bob
Member - WAFNC
1997 22 Classic (sold)
1997 Formula 271 Fastech
502 Mag MPI