A few sources suggest that mounting a foam pool noodle on the back of your bike can help stop drivers from passing too closely. Such noodles are available at my dollar store, unlike spring-loaded bike safety flags.
But I fear that both the flags and the noodles may be illegal.
Anyway. I am considering carving such a noodle so that its tip tapers to a "sharp"-looking point. (Pool noodles are hollow. I guess I would carve the noodle asymmetrically.) I would mount it like in the photo, except that I would have it stick out less: only a couple feet (30-50 cm) towards overtaking traffic. I'm not sure how I would mount it on the bicycle. Maybe I'd use a bungee cord.
Still, imagine that a sadistic driver used his car to push the carved point towards the edge of the road. I'm no physicist, and am not sure what might happen to my moving bicycle. What are the possible outcomes? (Note that if the noodle was illegal, and if my entire bike was damaged, I might not even be able to get compensation.)
(Optional:) Also, in general, do you think it'd be safe for me to mount such a noodle on my bicycle?
(Optional:) Do you have any alternative suggestions which are still frugal, but are more safe?