The nipple is all you need. Bore a hole the size of the O.D. of the nipple pipe in the fusor body and just push the nipple through to the inside from the outside. Leave about 1/16 of nipple or pipe body extending into the chamber. TIG welding will use this as filler metal to fuse to the inner chamber wall at the hole.
You will have to jig or hold it in place while tacking so that the nipple remains normal to the outer surface of the chamber.
I usually slightly underbore so I have to force fit the pipe nipple into the fusor body from the outside, then tack weld in three places, minimum, before taking the final weld 360 degree pass with the TIG torch.
Richard Hull
Created on Wednesday, May 09, 2001 12:32 AM EDT by Richard Hull