First of the air vents test fitted. I drilled a 1" hole with a step bit and then threaded a 3/4" iron pipe nipple into the hole. It threaded in about 2/3 of the way. These will be secured with 3/4" conduit nuts and bushing washers when final assembly is done.

(DO NOT USE GALVANIZED PIPE! Bad stuff comes off galvanization when heated. These are plain black iron nipples)