Had my smoke vault for a while, love this thing however it got annoying having to constantly adjust the gas settings as weather changed overnight (Brisket over night so ready for lunch/dinner the next day). I use an iGrill to track temps and to alert me if something is wrong. When I'm smoking a brisket over night I typically have to go out every 30-60 minutes to make an adjustment to keep the temp where I want it; whereas I only add wood about every 2 hours. Then there was the issue where the flame got blown out twice and filled my smoker up with gas for who knows how long.
I had previously bought the NG conversion kit for my smoke vault, I have a Weber gas grill right next to it so it was easy to T off and fuel both.
For the project/modifications:
I bought a gas fireplace safety valve (ProCom NVDWA1) and a mechanical thermostat from an old NG oven.
I disconnected the supply line from the smoker and ran it to the safety valve, which then runs to the new thermostat, which is then run to the original dial.
The safety valve has it's own pilot light and thermostat; if it sees temps are below a pre-set value it shuts off all of the gas. So far, testing has been great on this. The thermostat's temp settings are off, but it's off consistently so I can deal with it. So far, it's been holding temps within 2 degrees.
Mounting the thermostat was fairly easy, however I have a good friend that is a hell of a machinist that made a manifold for me out of a chunk of aluminum he had laying around.
I got lucky with the safety valve, I scored a brand new one on eBay for $25. The thermostat was $55 and the dial for the thermostat was $15. Went to the plumbing store today to get the line (verified it was rated for the application I was working on) and a few adapter fittings. That ran me another $50.
Once all the parts were together, it took me about 2 hours to get everything up and running and then mounted.
Edit to Add:
I know there are ways to do this electronically, however I wanted to keep this low-tech as possible. I'm a big fan of keeping things mechanical where appropriate.