Wouldn't just a regular ball valve work? Or are you wanting and electrially-actuated device? I know most ball valves aren't lockable, but you might be able to drill a hole in the handle and lock it through something? I had a plain-old ball valve on mine hidden indside the framerail--worked very well, as found out the hard way! Got half way out the parking lot, it died, and I though I had a major problem. Took a while to remember I'd shut the gas off.
