Weapons against cars?

Snub nosed uzi?

Rather than confrontation, there is Avoidance. Specifically providing the spacially challenged drivers with a visual aid showing the minimum acceptable passing distance.

It's simple, cheap and effective being white plastic pipe 15mm diameter and 1.5m long, attached to your seat tube, perpendicular to the top tube on the off side. Gaffer tape does this well and ensures that if it were impacted the force would not cause a loss of control.

Because it is odd, it gets noticed, so ensures that a close pass isn't possible.

It removes the ability to filter within the lane, but not in a cycle lane.

Once you explain it to other cyclists there's no issue passing you in secondary position.

Any impact with the pipe is non-structural and loud enough to be captured on video camera.

I hope this works for you. Ride safe..

Came up elsewhere but Mutually Assured Destruction?

The warehouseman at a place I worked at had a sort of sawn off baseball bat clipped to his handlebars which he used to tap the roof of any car he considered to be too close.

He looked like a Beserker with his long blond hair,  beard and physique.   Maybe you can get away with it if you look sufficiently intimidating.

I have been assaulted just for shouting at a close pass, simply shouting "Whoa!" in surprise 8s enough for a driver to stop and have a go verbally.

I have knocked on a window of a close passing car at low speed and I find it is a great way to nearly fall off.

Sometimes I've wished I had something like this... https://youtu.be/B6lrMQCymgA?t=97

If you want to hit someone's car, be ready for a physical confrontation, and alert to the possibility of a driver losing the plot momentarily and driving into you.

Don't start something you're not prepared to finish.

Added to which, it is likely to weaken your position in the eyes of the law (even swearing does!)

Close passes are scary and frustrating, but sadly part of cycling.  Other than that, slowly shake your head, take a deep breath and enjoy the fresh air.

I recall reading somewhere that a spark plug can have an interesting effect on glass.

Just don't. Not worth either the potential retaliation from someone driving a bigger weapon than you can carry or the legal trouble you could find yourself in. Your plan also assumes that your driving antagonist cares about a dent in their vehicle.

Stick with the camera and stay squeaky clean yourself, including swearing and not breaking any traffic rules yourself.

What everyone else said. Sometimes you have to stand up to bullies - but if you chose to I'd strongly advise picking your moment.  That's not going to be when you're cycling (or walking, in wheelchair, scootering ...) and someone else is in a car.

Also if you escalate the consequences are completely on yourself.  More than that - are you willing to provoke what could be a life or death situation in which whatever happens you'll be in the wrong when the law finally gets round to sweeping up the mess?

If you feel this will deter people I think you're mistaken.  If you want to stick it to the man there are campaigns (for legal changes, for better infra) and direct action (Critical Mass) you can channel your energy into.  Just getting on your bike and riding it is righteous in the UK.

A hard stare?

Get a camera, back off, and report. The last thing you want to do is escalate and get yourself into a fight with two-tonnes of metal.

A lot of drivers are lunatics - I've had a guy get out fists swinging before now, just because I had the audacity to shake my head after he cut me up at a roundabout. If you go out and intentionally escalate every bad encounter with a driver, it won't take long before you get one of these lunatics, and you'll probably be dead before the year's up. Just don't do it.

I remember reading someone saying that hard boiled eggs were useful...

Ignoring that, I would say use particularly harsh language.

Anything that actually contacts the vehicle and risks damaging it, even if it proves that they were too close, could leave you open to a compo claim or charges.

IMO.  IANAL.

My experience is that the best way to make a driver angry is to touch/hit their vehicle and I wouldn't recommend it as there's a lot of bottled-up rage out there.

Some people use pool noodles to enforce the recommended minimum distance:

Personally, I don't like the aesthetics of that and it'd get in the way of my egregious filtering at times, so I just go for running cameras and let the police deal with the worst offenders. It's much better if you don't allow the bad drivers to live in your head rent-free.

However, on occasion, I have daydreamed about wreaking vengeance on drivers and I reckon a decent lock would be the best choice - heavy, damaging and completely plausible for a cyclist to be carrying. Also, don't bother searching for Ninja Rocks on your own computer if you intend on using them.

I would advise not equipping yourself for this purpose, you could end up far worse off if a driver takes umbrage and retaliates with a swerve, or gets out with a bat. Get a camera and report them instead.