Tony,
I used to spend hours filing up contoured blocks to fit between the barrels. Then a local British-trained gunsmith showed me how THEY do it. He takes an empty tin can and a pair of tinsnips, cuts a one inch wide strip out of it, about six inches long, and then starts folding it over and over until he achieves the necessary thickness. The surplus is cut off and voila! I couldn't believe my eyes, but that's how it's done.