I used tin foil and would make it into the perfect rectangle to fit on top then I would wrap the tin foil in saran wrap. It kept my humidity ok and I got almost perfect sheds. Then I got the idea of using press and seal saran wrap.
Its a lil more expensive but it will make a perfect seal and if you take your time it wont look that bad either. This is how I got perfect humidity before I switched to a racking system.