I'd like to know this as well. I have an armrest that has a hole I need to plug and would like to know what to use to fill this with.

The next best thing that I could think of is using a scrap piece of plastic, cutting it to the size of the hole, and using that scrap to plug it up using modelers glue (it 'melts' the plastic, so it's kinda like welding the plastic together).