There made from plastic,, there’s a product called “rapid fix” that I’ve used with great success.. its a super glue with a bonding powder ..you can build it up from both sides ( or just the inside) then sand it down .. it’s sold on ebay but you can find it at some auto parts stores.. just lightly sand surfaces and clean ,,, make sure it flows down in the crack..i bought my 1st rapid fix off the snap on tool truck but not sure if it’s a snap on based product or just something my dealer had on his truck and was selling…I have bought it from pep boys but around here they no longer sell parts ,,,they only do service work..it works really good on any plastic..