Doors and walls are now complete!
There has been a lot of progress. I am sanding away as the doors and walls are now complete. I am really impressed about how tightly everything is fitting together. I purchased a tube of wood filler and have only had to use it on one joint so far.
Each of the sidewalls has a single plank running down the length of the center, instead of twin doors joined by the center support. I believed it would lend a lot of strength to the walls, and it does seem to be the case. I have a 20 degree bevel cut into all the door panels... looks great and that magic angle on a 3/4 inch board adds exactly 1/4 inch to the width of the board, so it makes measuring and planning quite easy.
Also, I have been joining them using a plate joiner and #10 biscuits. This way you have a lot of strength without needing screws everywhere.
Once the door frames are assembled, I attach them to some pine boards (lighter and cheaper than poplar and will not be visible) for some additional support.
The main doorway needs a solid frame, and with a whole lot of planning I have a decent solution. The design allows the hinges to be hidden, the doors to have a wooden threshold, and allow for enough strength to withstand a bit of pressure. The weakest point is a 3/4" x3/4" joint at the bottom, and I pinned and glued them together with a 3/8 inch wooden dowel.
I do not have a pic of the completed units with all the trim in place yet. I am starting to prime the walls as sanding is complete. Seeing a beautiful blue wall will make me very happy.
So far, I have spent about 30 hours of labor on this thing (4 hours here and there for the last month). That is solid labor...