The trick is to buy the right brand of valve. An Austrian manufacturer called Praher makes a very nice PVDF true-union ball valve with a locking handle. They make the same design in PVC and PP as well.
If the valve is already installed, you'll probably have to get something fabricated- a bit rough when the body hasn't got a place to fasten anything to it...I wouldn't recommend thermally welding anything to the valve body. You could fabricate a u-clamp type arrangement that entraps the handle and prevents it from being rotated- but you'd need tools to remove it.
What is the reason for locking it out? If it is simply to keep someone from opening it, a danger tag or an FYI tag indicating not to turn the valve is what we use. This may be a little less than what looking for, but it will suffice. Otherwise I would have to agree that a U-bolt design would be best.
Follow the links to valves and flow control products. They have a lock out cover that goes over the handle and locks together. It is made for their valve, but I suspect it will fit others. I've see other people with similar designs...maybe McMaster Carr or Lab Safety?
