I opted to use the 22mm brass compression fittings so epoxied a nut onto the outlet;
My first attachment was the dog bowl method and used a Tank Flange Compression fitting in 22mm;
This proved to be unsatisfactory with the HeaterMeter not able to get decent control. I played with the PID values for two test burns but could not get to a happy control. I'm pretty sure the problem was the side entry of the blown air. Using Heat Beads, the pit oscillated by 30-50F and nothing I tried stabilised it. This is the problem;
So time to build an Airburner! Thanks Ralph Trimble for the idea!
I decided to go with 22mm copper pipe (nearly 3/4") and drill a hole in the bottom of the smoker. I've used stock copper fittings readily available in the UK so this makes the build real easy and the Airburner can be removed and the hole plugged with a stock end plug fitting. I used a compression Male Iron Coupler and a compression Female Iron Coupler to "bridge" the newly created hole in the bottom of the smoker. This inside the smoker;
And this outside;
Then onto the Airburner build;
The finished item;
A final 90deg elbow and a few extra pieces of 22mm pipe plus a 22mm Compression Coupling means I use the same nut that I glued on for the Dog Bowl;
I also used Nomex gasket seals on all sections plus selaed up the hinges with a strip of Nomex. They were a bit leakey!
Pit is now almost Rock-Solid. I'll have fun tweaking the PID values for months yet!!
3 hours later and still within a degree of 225F either way.
I might grab some gasket material for the stackers on my Excel if the love affair with my UDS ever ends 
