Milliput superfine is good stuff. Two part epoxy as mentioned above so needs a bit of messy mixing first. Can be smoothed with a wet finger but dries very hard (harder than the surrounding plastic) so care is needed at the sanding stage not to oversand the surrounding area.
I use 'green stuff' I expect this is similar to the Tamiya filler you used, it melts plastic and therefore bonds very well. Can't be smoothed when wet so the trick is to apply it in a tidy and accurate way to start with, I use an artists palette knife which allows me to apply it to gaps without needing to spread a lot on the surrounding surface. Don't be fooled by thinking it's dried quickly ready to sand, it forms a hard crust but try to sand it and it will be gooey underneath, allow 24 hours before sanding.
Superglue also works well as a filler for small gaps such as along seems, can be dried almost instantly with kicker and sands at about the same rate as plastic. Produces a nice smooth hard surface, great for re-scribing panel lines. Almost all my filling is done with this product.
Si.