I wouldn't want sheet rock exposed to that level of moisture for long periods. It's basically chalk compressed between layers of paper. It'll absorb moisture and, when the temperature goes down (when you turn the heater off), it may start to mold. It'll definitely weaken over time if it goes through a lot of wet and dry cycles. I'm not sure I would want the wood exposed to the inside either for the same reasons. It would probably work for a while, though.
From what I know, there's three types of tobacco chamber you might want to build.
(I invite other members to correct my numbers if they're off. I've never done flue-curing. Much of this information has been culled from elsewhere on the forum.)
1) You might want a box you can use for yellowing. This would be low temperature (90 - 100 degrees F) and high humidity (70%+). You would use this to yellow the leaf before you hang it up to dry. (If it dies green, it dries green. Bad baccy.) This should only take a few days. Watch for mold in these conditions.
2) You might want a box you can use for "soft" kilning (my term for it). This would be medium temperature (122 - 128 degrees F) and high humidity (70%+). This gives the effect of "accelerated aging." This will take probably three to six weeks. As long as the temperature stays above 122 or so, you shouldn't have to worry about mold.
3) You might want a box you can use for flue-curing. This would be high temperature (165 - 180 degrees F) and high humidity (70%+). This gives the effect of "locking in sweetness" for the sweet varieties. (It's unnecessary for many varieties.) This will take probably two or more weeks. You shouldn't have to worry about mold in these conditions.
Of the three, the flue-curing chamber is the most demanding on build specs and energy consumption. The good news is that, with proper controllers, a flue-curing chamber can be used for any of the three purposes. The bad news is that you'd have only one chamber, so you can only do one of the three things at a time. If you don't have a lot of plants to deal with, this should be good enough for all your purposes.
If you can acquire the materials, I would definitely recommend insulating it heavily. If those are 3/4" pine boards, they should give you an r-value of around 1.25. If you want a flue-curing chamber, the temperature difference between inside and outside will be probably 70 or 80 degrees F. I would recommend 4 inches of XPS foam board, but that can get expensive. That would give you an r-value around 20. It may be that 2 inches would work. That would give you an r-value around 10. Somewhat surprisingly, the average refrigerator/freezer has an r-value of 10 or less. I think you can get away with just 2 inches, though 4 would mean less use of electricity.
(I think you might already know this part.) Running a kiln is all about putting heat in faster than it can get out. The better the kiln is insulated, the more efficient it will be. If the kiln is not efficient enough, you'll have a situation in which the heater stays on constantly, but your maximum temperature still isn't as high as you want it. If the kiln is theoretically perfectly efficient, the heater only has to get the chamber up to temperature once and never come on again. So, the goal is a well-insulated chamber that will hold its heat without running the heater too much. You'll be running this thing for weeks at a time, so efficiency is kind of a big deal. The power draw should be pretty low, however, so it's not a huge deal.
Recommendations (assuming you have the time and the money):
1) 2 inches of XPS foam board all around the inside (wood on the outside), caulked in with foam-safe caulking.
2) Some kind of heat sink to move the heat from your heating pad into the air more quickly. Pads are great for heating, but very bad at spreading the heat. You might be able to use a $20 space heater that already has a blower built-in. (Just make sure no tobacco or anything else flammable can fall on it.) That would seem like the better option for a box your size.
If you can do those two things to the chamber you already have, I think you might be ready to do the business.
