How well does Milliput adhere to styrene?

Yes, I was going to rough the plastic up a bit to give it something better to hold onto.

Thanks, I'll give it a go!

I've never had a problem getting it to stick, but I tend to use it for filling seams.  Should be ok as long as the weight isn't over the top.  You could always set up a test rig with some spare styrene stuck into milliput, then apply a weight on the styrene and see if the milliput gives way.  I suspect that the surrounding styrene would give way well before the milliput though.  Roughing up the styrene to give it a key would also help the miliput adhere.