If I needed to leave my boat outside in inclement weather, I'd probably invest in one of those prefab aluminum carports with sides. Another option is to contact an awning shop and get them to custom make a Sunbrella cover (or equivalent breathable fabric.) They're really not that time consuming to make. If you look closely, just about every medium and large size city has an awning builder/retailer.
My Bennington is 3 mos. old, and it came with the tight fitting factory mooring cover and 4 adjustable poles. It's always left hanging in a boathouse out of direct weather, so the OEM mooring cover is sufficient for our use. As much as new pontoon/tritoons cost, they will last so much longer if kept completely dry. My old pontoon boat's seats are finally starting to crack after 26 years in the boathouse uncovered.