I understand your frustration in locating the proper drum and or lid. To answer your lid size question, based on my experience lids from open head drums won't fit closed head drums without modification. (Cutting and welding). Trust me, save yourself the headache and find a open head drum WITH its own lid rather than playing the mix and match game. I mean, its possible but why dick around with it when you can use a drum with a proper fitting lid? also, the rolled edge of an open head drum provides better sealing of lid due to more surface area of sealing edge. Re: a diffuser- you'll naturally use more fuel as you're blocking much of the direct radiant heat by using one. Personally I feel they're only useful for very large, multiple rack cooks and even then not a necessity. A diffuser Just helps food on bottom rack not take so much heat to make it cook faster than the upper food, but without one this small issue can be managed. I think I've used a diffuser once or twice, in many years of smokin.