Are you running a mini secondary regulator or just one? I've seen those where you stick a ziptie through to lock it.
If you wanted to go dirt cheap, just get some PVC pipe that fits and drill a hole through it lol.NOW THAT IS STUPID SIMPLE!
Could make those fairly quick on my lathe to be honest!
I had an idea of something along those lines, but having grub screws on the bottom to grab onto the regulator, and taking off the knob to adjust it, and replace it with an allen bolt. Then have another hole bored through the top of the part, and have this threaded so you can screw through it and keep you from using anything to adjust the reg.