They are plastic but that doesn't mean they don't work. The big one fits well because the hole is round, the other two holes are oval, but the small plugs still fit, and no matter how you insert them, the hole is always closed. I only installed them last night so I have no idea how they will hold up to road vibration or temperature changes over time but as they snap and seal the holes as promised I'll give them a 3. I would give the rubber a 4 and the $5.20 Skinny Rubber is a bit pricey but…