I took some acrylic, bent it and siliconed a magnet to to it. Drilled a hole in the acrylic to mount the PH and another magnet for the outside and I have a magnet holder. That said, I am seeing some degradation of the silicone around the magnet. I have seen a recipe for a 2 part resin mold to make your housing with magnets of your choice. That will be my next project.
*180 gal Display, 100 gal basement sump, 33 gal refugium, 3x250 MH, 2x160 VHO actinics, zoos, some softies, LPS & lots of acros and other SPS.
*100 gal prop tank plumbed into main system w/ 2x96 PC lights and 1x150 MH,