Bogo
FOOF
I was thinking about supporting the top evenly. Take 4 2x4s of equal length with notches in the ends. One end would be put over the top edge of the base, and the other end under the top edge of the top lid. If placed next to the main uprights, they should support the top evenly. The other thing is the pop-up should have come with some sort of secondary hold up devices. Usually a one piece u-channel that goes over the inside of the extending uprights. One would go over each upright. Once in place the top is lowered onto them, and can't go down any further. The top could be lifted and set down to rest on them. That should also hold it up evenly. If the pop-up didn't come with them, it may be a good idea to make a set.