I think it's Topeak that has a really big cage with mounting holes to fit standard bottle cage mounts. (There are also probably 900 other companies that make something similar)
It looks like a large bottle cage. But it has a strap. So you can put a small bag or whatever on there and add some easily removable storage.
Or you could do a trunk bag / pannier. Just put all the standard repair stuff in there, and toss your lights in there too when you stop and boom...carry it away with you.
Ultimately nothing on the bike will be theft proof. If you make it super secure so it can't be easily removed, there are jerks who will just break it with a rock just because they can't have it.
Option B is to move back to Austin, where they still prosecute criminals.