I looked into these at one point, as my slideout does not have one. What I noticed is some (most) trailers have a track for installing the topper already. My trailer doesn't so I guess I'd have to remove the entire length of the rubber whatever it's called, cut it to size and put the topper in between.
If you look at
around the 1 min mark, you can see the track they use. I don't have one like that on my trailer so I'm not going to bother with one.