i bought a pair of those, not from the vendor above, and they don't fit my bolt size. plus i would need four rather than just two.

 

bryce has some very nice stuff, but with a minimum order of a few hundred pieces, i think i'll keep looking. i tried the tack weld approach but i ended up melting the plastic bracket, messing up the powdercoating on my bumper, and the bolt tip broke off easily when i tested it's effectiveness.

 

This may be a lot of work, but here goes.

Method 1
How about using a castleated (spelling?) nut, like the kind found on tie rods and ball joints? You could grind it round, instead of the hex shape that it comes in so that no one can put a wrench on it, next you could make a "key" to fit the castles by filing a second nut to fit into the first, you would only have to make the key once. As for the bolt, use a carriage style bolt, found at your local Home Depot or Lowes. This style bolt is rounded on top and has a square shank that keeps it from turning, it kind of looks like this (==.

Method 2
Another method I have used is to get a small pad lock, with the U part of the lock thinner than the bolt you are using. Next, unlock the lock and cut the U flush at the side that pivots. The U that fits into the lock is now separated from the lock.

This is a little hard to picture, so I'll try and explain it better. On a standard lock, the U shaped piece of metal is held in with a small notch, on some locks (the ones you will use here), this notch will appear (when the lock is unlocked) on the side that lifts out of the lock, the other side of the U pivots in the lock, and you will cut that side of the U flush with the lock.

What you will have left is a lock with one part of the U flush with the top of the lock and the other side, will have a hole. Next, you take the cut U and then cut it again so all you have left is a straight piece with the notch in it. Take this piece of metal, which should have a smaller diameter than the bolt you are using, and weld it to the end of your bolt. Grind away the weld, so that you can still place the nut on the threads. After you tighten the nut, snap your lock back over the notched end of the bolt. With this design, you can loosen and adjust your lights without unlocking them, but the light cannot be removed because the nut will not come off all the way without removing the lock on the end of the bolt!

This was something I invented after refusing to pay the outrageous prices for a trailer locking hitch pin. You can use this method on any bolt as long as you can find a lock with the U portion small or large enough for your application. Hope you can use my ideas!