This is a remix of @cushmail_454332 design for a Hepa filter. I did not have 5x5mm magnets on hand, only 6x3mm. It needs a total of 20 magnets.

It still has the small holes which you can you if you placed a magnet in the wrong orientation.
I had to move some a bit offcenter to allow acces, and i added 2 extra magnets for the lid.

The magnets are pressfit, the tolerances are tight enough to hold them in.

Print the lid with the recessed part facing up.

The main part will need support, i printed it with the largest face down.

Be advised ; the magnets may not be strong enough to hold the entire model in place on the backside. I cheated a bit by adding a small strip of tape on the inside of the model and securing it in place. Seems to hold up, and the magnets allow for easy acces and change of the filter itself.