The topper on my main slide is about as tight as it can be, but it does sag a bit in the center and will hold water. Found that out the first time we used it in chilly weather. The thin ice sheet that crashed down when I was retracting the slide almost made me mess my drawers. Welcome to 5'ers.
To remedy that, I copied another forum member's idea of using PVC pipe to construct a support system for it. Inexpensive, easy to construct, and it works. Comes apart and takes up very little storage space.
The other three toppers I have are shorter and do not pool water.