Gorilla heavy duty construction cement. Can be purchased by the squeeze tube or in large tubes that need a a caulking gun.

What kind of long term results have you seen with this adhesive?

That is kind of hard to say. On upholstery projects I hopefully I never see again. I have used it mostly for attaching velcro to hold the seat bottoms and backrest centers in place. It is waterproof. I suspect that on magnets to any hard surface should work well. For a test, I glued to bricks together and let them set over night. It took a hammer to break apart. The bricks failed before the glue joint.

Anyone interested in using magnets, give this article we wrote in 2021 a read: Magnets are the New Fasteners. Here’s What You Need to Know

In it, Todd Ramsey of Mobile Solutions told us that if you use their Magnet Fitment System, you actually don't need to use additional adhesive because the cut slots are so precise they make for a tight fit. But if you need to, CA glue works great on MDF and soft plastics.

"Since the diameter is so precise, very often there is no need for additional adhesive in the 'pocket' that’s countersunk, although some fabricators may choose to add a small drop of CA glue on applications like MDF or soft plastics. Harder plastics and aluminum have such great rigidity that the fit is very tight."

It might be overkill but I have usually used coarse sandpaper to rough up the side that will be glued to give it better bite

That's smart. Never hurts to make extra sure they won't fall out. Last thing you need is a client coming back over a missing magnet :lol: