Hi Tim,
I'm glad I can surprise folks with the use of glue!
It would be perfectly possible to hold the two lid boards together with the dovetailed battens, using a pair of pins crosswise at each batten where the lid board edges come together. The problem with the pins is that any expansion/contraction of the lid boards would crush against the pins, and being side grain in those locations, both the pins and the boards, the crush would deform the wood slightly. Then when the board moved again, the joint would be loose. There would be a possibility that the two lid boards could become open along their joined edge, allowing dust and debris to get into the space.
If the lid boards were thicker, say 0.5", then it would be more reasonable to tongue and groove them together- possibly even with a dovetailed tongue and groove. Even then I would still choose to edge glue the boards. Long sliding dovetails are tricky to fit well and in some cases can go together 80% of the way and get stuck. In this project, we use untapered dovetailed cross battens. Because the movement of the lid boards can deform or impair the fit of a tapered dovetail batten, the joint tends to work loose over time. If you pin it, then the joint will become loose by the seasonal movement crushing the long grain of the male dovetail slightly. With an untapered dovetailed connection, and glued boards, though there is a trick to fitting the parts together, the one-piece board can be pinned to the batten in the middle and that will allow for seasonal movement while keeping the lid boards centered to the battens and without any gap opening up in between the boards.
There is one solution which would allow for no glue: use a single board wide enough for the lid. A 14~15" wide vertical grain piece of wood is not impossible to find, but is not fair for me to specify. I've been making use largely of what I have one my scrap pile, so that lead to the edge gluing. It wouldn't be a reasonable expectation on my part that group members obtain really wide vertical grain material for this project. A 15" wide VG piece of wood comes from a tree at least 36" in diameter, and that sort of material is not available everywhere and can be quite costly. Not sure I'd want to use such material for a tool box....
Now, with the floor pan the situation is much the same, however the main purpose of the joint there between the partitions and the floor is twofold:
1) the partitions are there to stiffen the floor panel and are connected to prevent it deflecting down under load
2) the floor panel, by having a shallow dado cut in its top surface, keeps the partitions from bowing or deflecting sideways.
The connections are dovetail wedged pins, and there are two per partition. Seasonal movement could cause some slight long grain crushing of the floor panel mortises or the sides of the pins, however the joint is mechanically connected cross wise: the wedges expanding the pins push against end grain in the floor panel mortises, and thus the connection won't be degraded by seasonal movement. I expect the seasonal movement effect to be at an absolute minimum though by way of choosing to use VG material for the floor panels.
If study group members do not have floor panels with vertical grain, the potential for movement is greater and I will recommend they use on central wedged pin per partition instead of two. That way the boards can be kept centered at least.
~C