I am certainly not the person to answer this, but I would assume that your plan is fine, as long as all is sealed really well. The timber may not be the best insulator, and you may want some insulation material in between the timber and the sheet metal. Frankly, I started with a chest freezer, thinking that it should be more than enough - and considering how many seedlings you have in your other thread, I would say that the bigger the better. Already, my freezer is way too small, considering what I would like to do. And the height of the chest freezer is an issue, too, as I can only have one row of leaf, much of which extends to the floor. I would imagine that you need to determine how much leaf you wish to put into the chamber at any given time, in order to determine the size that you want to build. And then build it a bit bigger than you think you need.