> Or a cheap solvent.
>
> :-)

Ok, I'll weasel out of that by reminding you the OP was just
lending the circuits and that he'd spot any tampering

Having seen the "tamper-proofing" topic debated several
times I shouldn't have left "efforts as an unqualified "serious".
Hackers are always a step ahead and will go to any lengths,
even as an academic exercise. It's a matter of degree of course.
As is often the case, reasonable steps will deter most people

You could shield out X-rays with metal
You could put extra dummy components inside to mislead
You could put a vial of circuit-destroying acid inside
You could make connections from perishable materials (eg
conductive epoxy)
You could use code dependent on external conditions
You could add to this list

For a circuit that blinks an LED, not worth it

For a circuit that allows untraceable and easy access to money
or services or information, worth it. Somebody somewhere will
crack it eventually, but if you're smart what they've just cracked
is obsolete

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