You can cook more good BBQ in a 55 gal. drum than you will ever be able to eat!
I have made several custom builds. To make the holes I use an electricians "slug cutter". It makes nice clean 7/8" or 1 1/8" holes. These are perfect fits for electrical box plugs. I have also threaded 1/2" and 3/4" pipe nipples in the holes to draft air closer to the charcoal ring. The slug cutters are expensive tools but after you make a few cookers they are well worth the price.
These things can really cook.
Bill Napper. Team Cook "JoeJo's BBQ"
Custom offset stickburner
Gas assisted cabinet smoker (400 lb. capacity!)
55 gal. drum smokers galore! (All custom built)