Its too hard to draw what I'm talking about, but I think you can get the idea. Make it so the tarp goes above ground so rainwater won't flood in. You could get something that sticks on 2 sides, some weatherstripping for windows or something. Make sure to seal between the 2x4s and the tarp. I don't know how much hinges and latches cost, but you could put hinges on 1 side, then have a latch on the other side to secure the top. If you want to really seal it good, put some more weather stripping on bottom of the plywood too.