EZ--the 1 by 2s measure 3/4 by 1 1/2. Which by itself is FINE for each spar. I and some others doubled them up because we're total nuts.
Like it was said above, all of your options above look fine. I just didnt think a cheap pressed panel for the inside would be enough, so I stand corrected.
The only other issue I can see is with the fact that you're not building a galley:
This means you won't be installing those bulkhead/galley walls, right? I just don't know how important those walls are to total structural integrity of the TD--it may be a lot. So you may want to ask around for people who built theirs without those walls, and if you don't put them in, you may want to overbuild other aspects a little.