using magnets doesnt work for rays as they move stuff about too much.
i use bread crates. .... no i didnt spell it wrong . i said bread crates.
the crates shops have bread delivered in. the drivers dont like picking them up and they are often left outside anywhere that sells bread. there are a few types but the one i use has a reasonably fine "mesh" im sure you will know whats suitable when you see it .
i got a few that were sitting around for weeks . chopped the edges off with a jigsaw and used tiewraps to hod them together.
if you want a structure you can add pvc pipe cut and drill it so that it reinforces the structure in a triangle formaton.
worked well for me and after "stumpy" i now always use this when im expecting pups and have not had any incidents since.