Try Squadron brand green or white modeling putty. IIRC I used the green when I filled some heat stamping and it works great. It does shrink a bit, buts sands easily and takes paint without a problem.
Auto body filler is another option. I used to have a large tube of the red 3M stuff and liked working with it but since the quantites I use are quite small, the tube aged and dried before I got my moneys worth out of it.
As always, you may want to test a couple of different fillers on scrap plastic and see which is easiest for you.
HTH
Jon