just make a box basically, then run the ends together, and use the clip tool to clip each corner of each wall diagonally, so they run together. and then you dont have the walls overlapping each other.
this is true, hollowing causes very high r-speeds. In order to have stuff in the drop down menu you have to dl this file called cs-experts.fgd, it puts into that menu all the options for what a entity can be, and what you can make solids into.