I printed a set of black and I'm trying that out, now. using a chrome lock so that it stands out a little. there are black locks, too, but if its dark, black on black on black seats makes it too hard to see. the point is to be seen but not look too inviting.
honestly, with regular hand force, I can probably yank it up or even break the cam on the lock (its printed plastic; but metal would be far better if I can get one bent in the small size it needs). but again, I don't care about locking the seats as I won't keep anything valuable in the trunk. I just dont want to come back to broken glass and have all that heatbreak.
a friend bought some plastic film to put over the triangle. its supposed to be smashproof. but not sure that is a great idea. how do you get the window out, if you need to, then?
and maybe if you frustrate the thief by making the glass just bounce back (with the film) he may be annoyed enough that he damages your car, just for spite.
hard to know how far to go to protect your car. I live in the bay area, so I can't just do NOTHING. I'm trying those 3d printed visible 'locks'. will report back in a separate thread if I get broken glass, later on, with these things installed. (note, I'm NOT going to tempt fate and I'm now rethinking about parking in this or that lot, if it looks too empty or too late at nite. damn. hate having to limite what I can do with my car, but I do NOT want this happening to me!)