I copied advice I found on this site.
I used black pipe nipples, .75 inch I think, in holes cut with hole-saw.
The pipe just screws into the hole but I anchored and sealed with conduit washers, after I burned the galvanizing away with torch, which was likely not necessary.
Black pipe nipple caps cover three of them, the valve takes care of the one.
The valve takes care of the one that comes up to waste level with copper pipe, which is just pushed together, anchored to side with strap and couple small SS bolts.
One covered nibble is barely visible in pic.
I might add that I am glad I used smaller drum. I only smoke for two, but if need arises, I use both grates.
The hole for the chimney was a PIA to cut...took 10 minutes, but that top was a lucky stroke of luck...thermometer probe is near perfect position when installed and squatchin' going, as in second pic from last April...
The SS chimney is near exactly the diameter needed to maintain flow with its area matching the sum of the vents...or close enough it does the job, no creosote smoke after it gets going.
Rarely mess with it once it gets to temp, save messing with valve once in awhile. Usually end up with two vents open, bottom opposite and some degree of valve opened.
Better many vents to begin and later shut, to me.
The chimney may be from Char Broil...forgot, some brand of burner...will send if you call.