Sounds like you're building exactly what
Magneshade has had for some time. We have Magne shades for our MH, and they are great. Provide great shade and shield out a lot of heat from the sun, yet you can still see out it very clearly. Very easy to put up and take down, too.
Sorry, I can't really provide you any advice on what kind of magnets to get, except to say that the ones Magneshade uses for same purpose you are talking about, they are REALLY strong. When initially installing the magnets inside, you have to be very careful, as they can pinch and hurt you.
I'd look at what a Magneshade will cost for your MH. Yes, they are not cheap, but depending on what magnets you buy, you may end up spending close to same amount doing this yourself vs buying some from Magneshade.