I too just bought one at Wallyworld, $120 I think, appear to be comparable to ones at West Marine for 2x the price however time will tell. I have bought cheap ones that lasted years and others months. I keep my boat covered under a 20 canopy too. The Florida sun is brutal though.
No doubt some more expensive ones will last longer just hard to tell. At some point you get deminishing returns for the $ though. Trailering is another ballgame though.
I think I bought the G size cover for my 1968 20'SF, think it said 19-21'. It does reach over the sides fine but you do have to stretch it. It was not specific for CC boats, even said "except some CC" but fit ok, will work for what I need. I have to keep a bow rod to prop up for drainage.
Heres a shot with the cover;
