In regards to your photo of the Fuller square and comment
"With accurate cutting and filing (or milling if possible) it is possible to get a very square and rigid frame, this can be quite fiddly to get right but its worth putting in the time to get it right."
put the Fuller square away, you already have a perfect 90 degree square in the build. Instead of trying to square the outside corners of the two extrusions concentrate on the inside corners. Use a Cast Corner Bracket instead of the less rigid L Bracket and you've already guaranteed that the two extrusions are 90 degree square with each other even if they aren't accurately cut on the outside ends. Those Cast Corner Brackets come out of the casting die perfectly square. It took me a while to realize that.

That is one solid looking machine! I'm thinking you could set this loose in the bed of a pick-up truck, hot wire to the truck's 12 volt system, and get decent prints motoring across country.