trailer frame

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?