[Ambientnoise]

You can get "plastic weld" from model shops which really does weld (poly) styrene and some other common plastics. It is a very runny liquid, normally applied by brush to the joint line of close fitting parts. It then wicks into place, partially dissolving the components at the interface to form a weld. You can dissolve chips of the material to create a more viscous filler. Then of course there is modelmakers polystyrene cement which is more specific but still useful.

Ken