Summary

I remixed the sturdy clip made by CallMeMO and used that to make a shelf that will hold cell phone, water bottles, etc. I made the shelf match the contour/angle (roughly 30 degrees) of our pool, so please make sure your pool is similar or it won't work.

I suggest printing the whole model at 30% infill, but adding a modifier for the clips and wall on the back of the shelf and print at 100% infill. I used ASA filament as it is sturdy and UV/chemical (chlorine) resistant. PLA will melt and warp.

When I printed the sturdy shelf clip, it requires infill for the gap. I noticed when I removed infill, some material remained which made the fit incredibly tight. So much so it caused the clip to split. To remedy this I used a Dremel to clean out the remaining infill material. I also took some material off the shelf clip to narrow the fit. For extra strength measure, I added epoxy to the joints where the clip meets the shelf. I don't know if you really need to go to all that trouble, but I didn't want my phone falling so it was worth the extra effort for me.