I use Go dies in a Sizzix Westminster big shot pro. I don't have to worry with the width but I can answer your question about a shim. Poster board. The cheap stuff from the dollar store that comes in a package of 10 sheets for 1.00. I usually use 2 sheets under the cutting tray. If the fabric is thinner I add a sheet. The board doesn't get cut, it is under the die/fabric sandwich. The Dollar General poster board is perfect because it isn't real thick.