I have a dremel and find that it is too fast (even on the slowest setting), too much friction, too hot = too much melting!! Personally I use a liquid glue that 'melts' the seams together and find that with a little fine sanding and patience in the 'joining' process, the seam is often not noticeable. If it requires clean-up, a little putty usually does the trick.
I do have a small rotary tool that I bought at a discount store that I use for 'rough' shaping. It was intended to be used for manicures, but I find it is just the right speed for working with plastic. It takes a couple of AA batteries and dremel bits fit right into the end of it.
There's lots of other methods too. I'm sure a few people will post some alternatives.
Murray