I would suggest you go to a plastics shop and ask them what glue to suggest. In the place in calgary they say they never use weld on they use a wierd chemical i cant remember the name, its very liquidy like water and melts the two together in seconds.
ya exactly the same thing as weld on 3 but not a brand name.. weldon 4 is the same with a tiny thickening agent.
the only reason they shop doesent use weldon 3 is they buy the raw chemical in bulk for a lot cheaper.