I recall reading somewhere that someone blew out the electronics in a Luxos light with direct current, not sure if it was a Luxos U or B. I suspect 5v is safe, but I would be hesitant to go much higher.

Since the goal is to bolt on a light to avoid theft, any reason that you don't use a battery powered light and firmly attach it with a hose clamp or some other way of attachment that requires a tool that would take several minutes to remove?

Last summer I saw on Ebay a really cheap light (attached with rubber band, not bolted on) that was powered by USB. Took a month to arrive (shipped from China), but it worked pretty good off a power bank. It has a flashlight type beam, just as high as it is wide, but what do you expect for $5 USD?
https://www.ebay.com/itm/3000LM-XML-...t/201700089127

That Ebay light, I bought it to run off of a Sinewave Revolution, it did not work so good on that however. But for your purposes, something like this one that is held on with the rubber band could use zip ties instead or something else that would be difficult for a thief to steal.