Ian have a look at FPI Protostars blacklight tubes. 27gr/cm weight and the flocking is extreamy dark.
My 10'f3.8 Astrograph comes in at 11kgs total but that is with a conical mirror.
The 1200mm long tube weighs 3.28kg. It's very stiff and with a small amount of body filler to fill in the spirals and a nice shinny appliance enamel paint job looks a treat.
http://www.fpi-protostar.com/blite.htm
Mark