I made a trussed aluminum ridge pole for my 20 footer. It goes from the top of the windshield frame back to the top of the transom. I spread the canvas (sunbrella) cover over the boat and have sewn in tie down tabs every couple of feet along the perimeter. After drawing up the cover waist band snugly, I tighten up the side ties (to either the side loading rails or down to the trailer frame, until the cover is taught.
This works very well for rain but I would not want to trust it with a snow load. I suppose I could have incorporated a mid span vertical support to the floor, then a heavy snow load would not collapse my ridge pole. If you then throw a big costco tarp over that, the snow would slide off. At minimum it would make it easy to tug on the blue tarp and get the snow to slide off. You can see the ridge poll (sans cover) in this picture. I used this for years before I got the costco canopy.
This is another option that works well for me now. I still have to go out in heavy snow and push on the underside with the backside of a push broom, but the snow slides right off when you do it. With a fresh cover, I have had as much as ten inches of snow on it without collapsing. You can see all the snow that I knocked off the canopy, down on the ground to the left of the boat.
