On Monday, October 30, 2017 at 8:30:05 AM UTC-4, Colin Roney wrote:
I am considering modifying my glider trailer to incorporate
an extended fuselage support frame to permit rigging on
the lines of the modern lift-top trailers.
The frame/channelling needs to be about 2m long with a
section of about 60 x 50mm with tapered sides. Has
anyone had a go at making one of these frames and where
to obtain the parts? Is the chanelling in aluminium or some
other metal mix and are there any designs available?

I constructed one of fabricated aluminum and found it to be less than satisfactory.
Having had a trailer that had a track/frame like you describe I would suggest the following:
Steel channel tracks about 50mm wide with depth of sides of about 25 mm. Wall thickness 3-4mm. In the US these could be produce from 2 pieces of 1 inch steel angle having 1/8 inch wall thickness and welded into a channel configuration. Cross members would be same wall thickness and larger section.
Good luck
UH