Aluminium Warehouse do 2" and 2.5" 1/4" wall round tubing but only in fairly long lengths - not sure what clearances / roundness you'd get but skimming the inner should be easy enough..

Many recumbent cycles use this sort of arrangement for attaching the bottom bracket - I have a vague memory that some sizes of standard tube work well but can't find any references. ICE might be able to help you out Recumbent Trikes - ICE - Home - most of their manufacturing is done abroad now but they might have some bits for making prototypes.

I made a carbon outer tube for a recumbent by wrapping it round the aluminium inner tube and then wrapping that in insulating tape (to expel excess resin) - works well but not too great a finish. Cure it warm and put it in the freezer to separate (there might have been a layer of cling film in between as well - it was a while ago) - glass fibre would work ok and be cheaper.

I'd be tempted to use collars on slightly thicker tube unless you are being very weight concious.

Matals4u has aluminium tube in shorter lengths - Buy 3"od x 1/4" wall Aluminium Tube from Metals4u 0844 800 0069.