I've always wanted to find a way to attach ears to a wig seamlessly without ruining the wig. I've used barrettes before to make tanuki ears and they've worked pretty well (Well, they slipped off my head sometimes by accident!! But otherwise, they worked!)
I used upholstery foam to make a base for my ears, and then slipped/hot glued the fur onto the foam base. But I'll definitely keep this tut in consideration!
My ears are very heavy, because the fur is very thick and long. Plastic is not heavy enough to keep the ears. If I would use plastic, the ears would fall off the head. You can use plastic for smaller and lighter ears.
The screws don't hurt ^.^ because the screws are securely inserted into the metal. There is no part that sticks out and could hurt you. I wear the ears themselves since 4 years on various conventions