In the end I decided to work on the base first. Using a piece of walnut of approximately the right size, I planed one face flat, using winding sticks to check for twist. I then planed one long edge square to this registration face to create a registration edge.
Then using a marking gauge I scribed a line from the registration edge for the final width on both faces, and planed down to create the opposite edge.
Then, again with the marking gauge, I used the registration face to scribe a line along both edges of the board for the finished thickness, planing down to create on opposite face. This is the way that I dimensioned the sides of the box, and I’ll do the same for the lid.
With the base dimensioned, I rounded over the edges. For the short edges I was planing end-grain and so to avoid breakout I angles the plane at 45° in both axes.
The long edges are planed with the grain, so the angle is only necessary in one axis. This video from Paul Sellers demonstrates this.
Then came the glue up. Working quickly, I used a spatula to apply a thin film of glue to the bottom edges of the assembled box. Positioning the box onto the base and clamping each side, I left the glue to dry overnight.