I'm starting to build an atmospheric this weekend, copied from the ones we used in a class I took with my daughter in May. They worked well in class, get about 9 hrs out of a 5 gallon bottle of propane.
I'm definitely NOT speaking from any extensive experience, but the man who taught our class, Gordon Williams, uses them and he is very good at what he does.
I'm working off of a rough sketch he made right now, but I promised him I would draw up up a detailed CAD drawing for him to pass out in his classes. I'll pass it on here when done if there's interest.
They're relatively simple to build, mostly welding, grinding and drilling, and I should have under $200 in it when it's done. (I have a pretty good scrap pile though....)
These units are made from 10" dia. tubing, 8" long, not real big, but good enough for what I want at the moment.