To get the same holding power using steel plates or washers, you'd need more pairs. For example, you might need 12 magnet/steel pairs to get the same attraction force as 8 magnet/magnet pairs (these numbers are assumptions, I don't know what the real numbers would be)

So in the above example, you wind up drilling more holes in your lid (12 instead of 8 ), and having to mess around with more fillets in the lip (12 instead of 8 ). I can tell you that drilling those recesses in the lid to install the magnets was a hairy operation. Not difficult, just nerve-wracking. One slip and you could go right through. So why not achieve the same holding force, with less work? And less risk of screwing it up?

Polarity does not have to be an issue, depending on your building method and sequence. It would be physically impossible to get the polarity reversed the way I'm assembling it. Therefore, no worries.

: Posts I've seen on magnetic hatch fasteners indicate that people are mounting
: 2 magnets at each location. Why not use 1 magnet and a metal plate? those
: rare earth magnets are certainly strong enough and you don't have to worry
: about polarity.

: Am I missing something? I would sure like to know about it before I go ahead
: w/ mine..
: James