Here's a fitting that could get the job done without a custom weld. From St Pats (picture at the end). I would not buy their expensive stone for it, but get a pipe to female flare adapter and one of the cheap morebeer stones.
Or get the second one pictured and chop off the barb. Actually, morebeer might make one for you with omitting the barb. That second one is the stone that they put in their 'economy in line aeration' device and they were going to custom one of those for me before I got one made from brewershardware.