I have found a very good alternative to expensive modeling putty that is cheap, eeasy to work with, and it's as close as your nearest automotive supply store.
Spot putty, which is used in auto refinishing to fill minor surface defects.
There are many brands available, and the cheap ones work as well as the expensive ones in this case.
It's great for filling seams, making zimmerit, and I have used it for reproducing some small parts( kit bags and such) , all for about 3 dollars a tube.
A little goes a long way, so a tube lasts a long time. As long as you remember to put the cap back on.
You can even use it to produce a cast effect by applying it and "stipling" it with the end of a round flat ended brush.

Happy modeling.