Yeah, the ones is use are plastic. Could plastic weld them but could still crack where they are skimmed. I'll probably go with them riveted on.
If you had them welded in and had the plastic weld raised, you could then sand/grind the plastic back to flush with no risk of cracking. It would be as if it was one bit of plastic.
We have been repairing some of our bike fairings and used this technique to avoid the use of any filler thus eliminating the risk of cracking.