Made my caul tonight. I don't 
plan to make any bows longer than 64". So that's its length. I use 6 and 12" bar clamps. Can bend 2 at a time  on top. I offset the boards  so of I just want a straight stave with no bend i can do that also. Needs a bit more sanding on the top. I'll get that done tomorrow.  This gives 3.5" at 64"