My son caught the blacksmithing bug while he was at summer camp so we investigated building our own blacksmith forge and found that brake drum forges are a very popular way to get into the hobby. We built our blacksmith forge out of a brake drum, 2" pipe, cinder blocks, a light dimmer switch and hair dryer. Here is a video with all of the same pictures in this instructable plus a short video at the end of the forge in action.
This is the basic design. A 2" floor flange was bolted to the underside of the brake drum to which a 2" coupler was used to attach it to a 2" T-connector. A longer coupler was attached to the side to accept the hair dryer and a bottom pipe was attached to the bottom to collect the ash. This is usually capped with a 2" cap so the air can't escape that direction but I didn't want to have to wait until the forge cooled to unscrew the cap and empty the ash and I was concerned the cap would become permanently attached by rust since the forge would be stored outside in the rain. I'll show you how we handled that part.
Note: The brake drum was free but it took visits to several garages to find it because brake drums are harder to find now that disc brakes are becoming more common on the rear wheels of new cars. A large brake rotor with a large hub will work but the hub will still not be as large as even an average-sized brake drum.
The forge was designed to be very basic and modular so it could be broken down easily for storage so we just used three cinder blocks for the base.
Instead of capping the bottom pipe, we used a paint can filled with water to catch the ash and keep the air from the hair dryer from escaping through the bottom.