Mississippi State Uni has the instructions to make 2 different types of still-air incubators (styrofoam & plywood):

I have the Bellsouth100 (100 large eggs capacity)
Over the $200 mark though. After losing alot of eggs due to a faulty thermostat, I ended up using only broody hens for this spring. Have more chickens than I should have right now. :p

I always use broodies to raise the chickens, & turn off the incubator before the storm season begins. Sleep better on stormy nights that way. wink