This is What I ended up with.
Step 1: Overview
You can see the basic idea; build two halves that are 31" x 85", the extra three inches are to make room for the braces at the head and foot to reduce sag after the two halves are bolted together. The slats sit loose on the frame, and are connected to each other with a nylon strap stapled to the bottom so they can be rolled up and moved as one.
Fully dismantled there are five components, but I haven't decided on a headboard yet, so that may be added later.
The frame is all pine to reduce cost because you don't see much of it anyway, and facade is oak.