soldering iron isnt hot enough mate.
used a 1500watt plastic welder. selling it now 155 quid.
just used that really then skimmed with plastic filler. them loads of coats of primer then hit with a solid colour.
would post a guide but tbh dont really want ever tom dick and harry having one coz then looses the uniqness ive only seen about 4 done, top cat being one.