As mentioned above, swimming pool companies will be able to tell you where to get large sized visqueen although I think a tarp will work better.
As I mentioned earlier: A fellow club-member has found swimming pool covers to work nicely. We just ordered one 40' by 16', "medium" duty, for $88. He uses super-balls, short pieces of line with loops at each end and adjustable bungee cords to hold it down. He uses those foam blocks shrink-wrappers use to keep the lines from chafing the hull. He puts a line all around just under the rub rail and keeps that tight with a bungee, to keep the wind from getting under it and blowing it out.
There were photos of someone's emt structure here last year...he was a tube bending whiz!
You're probably talking about TrueBlue. Here are the pictures: I designed and fabricated an EMT
And here's the thread, from last year, in which I found the link: Winter cover up
You might also want to have a grommet making kit on hand; that way you can add holes to your tarp where they're needed.
If you use grommets, make sure to duct-tape the inside, or they'll do Evil Things to your hull, no matter how well you think
your tarp/lines are tightened. Ask me how I know
. Plus, grommets are prone to ripping out.
Better to use the super-ball method or grip clips.