Included STL is adjusted against https://www.mountfield-export.com/pool-set-azuro-var-403-dl-5-5-x-1-2m--wood-without-liner--ladder-and-filter--with-off-axis-holes--in-2-boxes/ for near-perfect, well snug fit on top guard rail. Hooks are used to hang various pool toys, diving stuff, etc. 

Claude assistance has been used for creating parametric SCAD file. This allows to modify this hook to any measurements/pools which has top guard rail.