What you're describing is no different than the glycol chillers used in most
breweries today, but you want to replace the unit that does the cooling with
a Peltier device for greater efficiency. I've considered this many times in
the past as well but never bothered to build the beast.

Also, you'll want to use a pump to recirculate the refrigerant to maximize
the chilling for the amount of stainless tubing you use. You don't have to
use a pump, but if you rely on the action of your refrigerant to do the
circulation for you, you won't get nearly as much chilling power for the
amount of tubing you want to use.

To deal with your other issues regarding the carboy. It's always been my
thought to build this chiller using a plastic pail or stainless steel pot
with holes cut in the top with the two lines from the chiller going through
the lid, with the lid suspending the weight of the chiller. Again, not much
different than commerical glycol chillers.

Cheers,
Scott