Instead of inserting the end plugs into the top and bottom of the mast, I decided it would be better to glue them into position when fixing the side panels to the bottom piece.
After spending an hour or so refining the shape of the front and back pieces, I glued the side panels to the back piece. Since the back of the mast was dead straight it was easy to line up the side panels with the back piece.
All this was done in the kitchen, where I placed the pieces on the upright backs of some chairs that had been covered with plastic bin bags. After protecting the floor with newspaper, I applied epoxy to all surfaces to be joined; then I thickened the remaining epoxy, by adding colloidal silica, before over-coating the same surfaces with the thickened epoxy. Clamping the side panels to the bottom piece was a messy operation, because some of the epoxy squelched out as I tightened the clamps, but I made a point of not tightening the clamps too hard to avoid starvation of epoxy between surfaces.
An advantage of doing the job in the kitchen was the warmer temperature, which made it easier to mix and apply the epoxy. Outside it was a little too cold and the air was somewhat damp.
It seems to me that I shall be able to make the frames, cleats and floors in the garage prior assembling them in the kitchen, but before that, I need to finish the mast, with its light and internal flex; then the yard and boom. I’ll not be able to start the boom until a new piece of Douglas fir arrives from Robbins, because the piece they supplied was too thin by 7 millimetres, but all the other planed wood was excellent on all counts.