The AirStick microsuction tape I was using to attach the box to the guitar back has stopped working after a few months, and the stack of 5 magnets I had inside the guitar (each magnet is N50 and 25mm diameter x 2mm thick) wasn't strong enough to compensate for the tape failing.
So I added another 25mm x 50mm steel plate between the two steel plates already on the bottom of the box, and put a second stack of 5 magnets in the guitar.
One stack of 5 magnets is North side up and positioned to attract the top steel plate on the box, and the other stack of 5 magnets is South side up and attracts the bottom steel plate on the box.
I stuck the magnets to a corrugated cardboard strip so I could keep them separated from each other, and to allow for easier placement in the guitar (a bit like the X-brace of the Tonewoodamp but with only 2 magnets).
Those two stacks of magnets are strong enough to hold the box to the guitar with no suction tape at all.
I actually put suction tape on all three steel plates, but have left the protective cover on the tape so the tape is not actually doing anything other than stopping the steel plates from scratching the back of my guitar.
So I correct my previous findings and say that having two stacks of strong magnets in the guitar and steel plates on the bottom of the box is more important than the microsuction tape.
I still recommend microsuction tape to attach the exciter to the guitar back.