That would work just fine. Unfortunately, it is difficult to do a dado on the angled side of the end piece. just doing it on the straight (top) side would work though... Make sure you leave extra material on the longitudinal form to it does not end up too low after it falls into the dado.

I think most people do not do so, because there is not much stress from side to side on the end form.

On my latest Guillemot, I screwed the 1 foot forms to either side of the end form about a quarter inch apart... that was close enough. I also screwed the form #2 to the end of the end form, then just slipped the thing on the strongback and used a couple little blocks of plywood to screw the form to.

As I reused the strongback from the auk for the guillemot, I am somewhat glad that I did not have a dado in the top, the poor form is thin enough at the ends.

Rick