Can't say enough about Baker Creek. Top quality, amazing germination, great service, and fair prices.
Another is Mountain View Seeds (www.Mvseeds.com). Thier annual border is one big smile.
That's the least of it. They are also bagging up the leftovers of what qualifies as fertilizer. Read "Fateful Harvest" cheap on Ki del and a wake up call. Heavy metal.
You don't need pressure treated framing lumber, only the sill plate next to the concrete. And, anything below ground. Moisture is what rots it. You also run the risk of off-gassing chemicals if you use it above ground. Just a fyi.
Be mindful of whether or not it's waterproof if it will be near water. There are nursery heating coils available. Idea for starting earlier outdoors in the spring. I think Farmtek(.com) may have some. I'm in somewhat the same spot and hoping wrapping them in plastic works. At least the end of...
Seems to me...you have reduced the volume of "unoccupied" air in the jar, so the humidity would increase. There's a mechanical engineering term for this, but I can't remember enthalpy???