I am attempting to make an optical chilled mirror dew point hygrometer, and
then a full weather station to accompany it, anybody built similar items?

Doing it the hard way, then! I would have thought the most important
thing would be to have a very smooth highly polished surface on a piece of
copper that is as thin as possible, say about 10 thou or 0.25 mm. It might
buckle if you bash it, though. Brass would be nearly as good, and easier to
work.
What liquid are you going to use? Petrol would be OK - very volatile,
but it will become a little less effective each time as you blow off the
volatiles, but cheap enough. I use it to top up a Tippex bottle. Pongs a bit
but the volatility is right.

Best of luck - Tudor Hughes, Warlingham.