I don’t know why it didn’t occur to me before, but all I needed to get the frame rails at the correct height in the jig was to find two old top bars and cut off the section opposite the wedge so they were the same thickness everywhere. And now the jig is all done.
The boxes are initially assembled upside down, then removed from the jig to fit the top rails.
And the rail height comes out perfectly.
Having made the jig I obviously cracked on with making up the six brood boxes that have been taking up space in the bee shed for ages. And I didn’t have to use a square or tape measure once 😀