Model Maniac - in a pinch you can make your own putty, here is how: take a small bottle, one that used to hold nail lacquer is best. Clean it with lacquer thinner, and then put some lacquer thinner inside. Then take some sprue from a model kit and cut in small pieces. Put the pieces of sprue in the thinner in the bottle until the bottle is 1/2 - 3/4 full with the mixture of lacquer thinner and plastic particles 1:1. The plastic should dissolve in lacquer thinner and in about 24 hours give you a bottle of very nice seam-filling putty. It dries slowly - needs about a day to cure, but has perfect adhesion to plastic, doesn't shrink after sanding and sands very smooth. And is very cheap, too!
Hope it helps and have a nice day
Paweł