Ours also collects water. I've tried throwing a rope over the topper and affixing in on each end which tends to form a channel for the water to run off. However, I'm also concerned that this will stretch the cover which creates its own problems.
I really like the idea (mine) of a perforated pvc pipe just slightly longer than the width of the cover. The water flows into the pipe and out the end. See stretching note above.
However, when all is said and done I just let it pool and it runs off when I retract the slide. No need to carry any more `stuff' for a sometimes problem.
Dick_B
2003 SunnyBrook 27FKS
2011 3/4 T Chevrolet Suburban
Equal-i-zer Hitch
One wife, two electric bikes (both Currie Tech Path+ models)