Looking at my steel catalog, the closest fit I could find for a CETME-based receiver was 1-1/2" Schedule 80 steel pipe. That's 1.90 OD, .200 wall, 1.50 ID - just bore the end a bit to accomodate the trunnion, and turn the OD to whatever strikes your fancy.
From what I've gathered browsing the H&K fora, the G3 and CETME use plain soft steel for the receivers, about .060-.063 thick. So you could turn the pipe OD down on the lathe to 1-5/8, or whatever you felt happy with.
I was at the steelyard today getting some other metal, and asked about the pipe. Not everything in those catalogs actually exists; or more properly, it's a "standard size" only if you'll commit to buy a railcar-load after they set up to make it. It turns out 1-1/2 Sch 80 is a very common size, and they had a bunch in stock.