the tube is solid. I use the scope for planetary imaging, so even minor faults in the figure would be fairly obvious - I have had some problems with astigmatism from edge supports, but have never felt that thermal distortion of the mirror shape was a major issue. Getting the thing to track the ambient temperature (and minimize internal air currents) is the biggest issue and I use a combination of aircon + Peltier cooling before use and a fan during use. A thin mirror would help in that regard and Pyrex would also be better at shedding heat - but the current system works OK. If you haven't already checked it out, the following is worth a look:
http://www.cruxis.com/scope/mirrorcooling.htm