Get yourself a sheet of gasket paper and lay it or glue it to the face of your diff/Gbox (use a kids glue as you want to unstick it) Then with a ball hammer lightly tap around the edges of your diff/Gbox.This will be enough to cut through the paper and will fit perfectly.
I have used cornflake boxes in the past but any breakfast box can work.Not because I'm tight or anything,but a 45 min drive for a sheet of "special" paper seemed a bit much.
If you use the "cheapskates" way maybe a light smear of silicone just to be sure.
What car? the gearbox to diff gasket on the 4 speed is part of the bearing shim calculation. You need the exactly right material otherwise its going to whine. Note sure about the 5 speed.