Hi Richard,
It'll be interesting to see your results.
have you seen this?
http://ghonis2.ho8.com/rebelmod450d16d.html
you might find some projects on this forum as well.
are you planning on forced cooling or just radiative cooling?
it'll help if you use two fans, one for the hot side and heatsink and one for the cooler side. should also help if you keep the inner area as small as possible, insulated and sealed.
i wonder if there are risks with condensation on the electronics.
good luck.