So you're talking about a full cover and not bow/cockpit covers?
To be truly trailerable it's going to have to fit perfect - nice and tight. It also has to have to reinforcment in all the right places, such as the windshield. Otherwise it's just going to billow in the wind and wear out. Your best bet is going to be to have it made locally - find an upholsterer near you with experience with making covers. There's more upholsterers out there than you might think. Consider having a ratchet strap put in the helm - you won't need any other tie-downs with that. You'd probably just need two poles.