What is your budget, roughly?
Are you wedded to any particular optical design (Newtonian, CDK, etc) and any particular mount (alt-az, GEM, fork …)?
Why does it have to be a closed tube? I used a Beacon Hill 18″ Dob very successfully back in the late 80s with a fabric shroud to keep out background light. It had the advantage of no tube currents.
http://beaconhilltelescopes.org.uk/ are still in business and they still ship up to 12″ with a solid tube. You could ask about larger ones.