Any magnet strong enough to hold a BP accessory though 1/2" PVC is too strong to be safe for hobby use. I've used many neodymium magnets in gecko, frog and aquarium applications. If a human can pull it down, a BP can, and if they dislodge it there's a chance that they'll get some part of themselves between the two magnets.
Silicone will not adhere to PVC. Solvent welding is the only "adhesive" sort of way to attach to it, and that only works if the thing to be adhered is also PVC or a compatible plastic (CPVC or ABS).
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