I built a barrel heater upright out of a 55 gal maraschino cherry barrel that had the removable lid for my boat/fish cleaning/gardening shed....oh yeah, it also has a bar and poker table in it, 29X24. It has metal sides/roof, no insulation. It was 29 degrees out and the guys would tell me 'don't put any more wood in that heater', had 'em sweating and in their t-shirts...lol.
I cut the lid straight across leaving the front 2/3 as a lid and took the back 1/3 and welded it to the barrel for a solid mounted place to mount the stack. I then cut a 10 inch wide by 6 in tall door down at the bottom as a clean out door/air intake combo. I adjust the airflow by cracking the door open or closing it entirely, dirt floor, it's a shed so it's ok if a few ashes fall out. I used a thin cut off wheel to cut it out and hinged the cut out piece as the door. No, it's not airtight but you need a lil airflow to keep it burning. opened about 1/8 inch seems to work well. The top was reinforced at the seem/cut with 1" angle iron, heavy barn style T hinges for the top/loading door. The lid has the original lip to seal against the barrel and the angle iron to fit the opening part against the welded part seals really well. If it smokes a lil when you first light it, close the door a lil. It will make the few tiny cracks draw instead of putting off smoke.
I built a rack out of 1" rebar and heavy 3/8 thick angle iron holding my fire about 10 inches off the bottom. Problem was, with the rack that high I never did build up enough coals to have a bed of coals to put my next wood into. I had to keep it stoked to keep it burning, hence the guys telling me to stop stoking the heater...lol. The rack is basically like you would build for a fire barrel to produce coals for a bar-b-cue pit. So then I cut a piece of expanded metal to lay on my rack to keep my coals up high, can fire it less, coals there to get the next logs burning. It works great although it is not a heater for a wooden strucure. With a dirt or concrete floor, all would be safe.
I have a manufactured heater in my shop that is lined with insulating firebrick. It is very similiar to the one posted earlier in this thread, not the Vogelzang, but the other one. It works well as we don't have extremely cold weather here, but if it drops to the lower 30's, in my 20 X 30 shop, concrete floor, metal decking material sides & roof, no insulation, it can get chilli in there. I think I am going to get an old Vogelzang box type heater, older American made a buddy has and use it in my shop. The thinner walls put out more heat and I have a wood working shop to put the manufactured heater in.
So while you guys are deciding on what type heater to buy/build, if it is in an insulated building, an insulated heater works well. But in an open airy shop, a barrel heater produces more heat.
I knew an old timer 35 yrs ago that heated an old farmhouse, big rooms and high ceilings with a "trash burner" type heater. That was what he said it was called. The walls of the heater looked to be made out of the same material as the stovepipe and wasn't very much thicker. He explained that the heavier the heater, the more heat you lose. The thinner the heater, the more heat gets put out. Sort of like building the fire right out on the floor and only having a thin shield to take out the smoke. It was store bought, looked durable, worked well, but only lasted several yrs due to being extremely thin. My barrel heater is 2 yrs old and shows no signs of burning or rusting out. But my stack is designed so no rainwater comes back down into the heater...another important thought in designing yours too.