Getting the magnets to come together is a big part of it. I have six pair of 3/8" on my internal hatches, no backing washers, etc. Because of the dense weatherstrip I chose, the magnets stayed separated by approx 1/8". The hatches would stay in place, but I had no confidence in the seal or that they'd truly stay. Wouldn't use the setup at all on external hatches.

While fishing around for a fix, I dropped some thin washers on top of the magnets. This extended the field enough that they all clicked together and held tight. Now they're plenty strong for my use, and I'd almost trust them topside. This isn't the right way to fix or build 'em, but it does show that getting the magnets tight together makes a big difference. Field strength changes with the cube of the distance, so every little bit matters.