Unlike a regular birds mouth mast, which has a structure that's self-supporting and self-cantering, and thus keeps itself straight during assembly, my modified birds mouth doesn't. I've already constructed the sail track section from two pieces of Jarrah. This exercise in wrestling a pair of floppy staves on the floor of the hallway was what convinced me that I needed to build a decent bench for the purpose.
Here's the sail track piece sitting on the bench, with a couple of 3m planks clamped to the face of the bench as a straight-edge. There's quite a bit of wobble in the sail track.
VHF collinear antenna, embedded in the mast.