@DC
Nice links
I appreciate the CF rout would be the ultimate material but from my limited experience of working with fibreglass i can see no logical reason why you could not build an exreemly ridgid gantry, the project i tried to construct was a curved concrete mould, but unfortunately i could not get sufficient vacuum on the mould and it resulted in about a 400mm length being correctly cast, however this was both heavy and super ridgid, if you live anywhere near the Barrow in Furness area i would be willing to donate a roll of the FG for you to trial a gantry prototype on the understanding that we have lots of pictures and you keep us all updated, i am convinced this is the way to go, the only downside is the time it would take mould making ect.
Have a Nice Day