You could also go to the nearest Super cheap (they are always just around the corner where ever you live) and grab some gasket paper it works a treat.
It's a sheet of cardboard like material that's designed to be used as a gasket, just cut out the rough size you need, place it against the surface you need to seal and gently tap with a hammer, this will leave an indentation on the paper which you cut out to give you the perfect gasket. I use it all the time for that kind of thing and it's as cheap as chips.