Oh wow then my tarp idea is definitely not a good idea. That would make it worse in fact.

You'll definitely need ventilation with those temps. I'm no expert on vents, sorry.

On the roof though, someone else mentioned about automotive caulk and that's a great idea as it functions metal to metal. Most roofing caulk doesn't bond well like that. So that's a good suggestion. You preferably do want to cover the roof so that water does not sit in the gullies as that will be a problem down the line. Marine ply will work but in that heat I'd seal it with a heat resistant roof covering, and if you are DIY handy I'd pitch it to one side to create a run off. But be careful that you don't inadvertently drain water down the outside as that will again cause you problems later on so overhanging any roof would be preferable.

Edit: using a heat resistant roof felt would negate the need for marine ply. That's 4 times more expensive than traditional ply.