> I recall an article Don Lancaster did for Radio Electronics (early
>'70s) that used conductive foam to bridge the pads

That works if you limit the switch travel so that the foam isn't
crushed otherwise it eventually starts crumbling. But some foams
are more flexible and resilient than others and would last longer,
so look around. Solid black anti-static plastic film can be used but
its resistance is higher than for foam. It would take only a quick try
to see if it suits your application. Foam may be just a few hundred
ohms per cm so you may not need a very large pull-up. Useable
resistance will depend on the actual squashability and density of
the foam. IOW - suck it and see

If you can stretch to something a little more complicated (I don't
know if this is for a one-off or production) you might consider a
magnet and reed switch (although I think some require a minimum
contact current). I have some very nice big push buttons scavenged
out of Toledo butcher's scales that use reed switches and work
OK with signal voltages and currents

Or even a tilt switch if there's room

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