The printable version is no longer supported and may have rendering errors. Please update your browser bookmarks and please use the default browser print function instead.
Take a piece of 1/2" thick plywood and cut it to 6" long by 3" wide. Cut a hole 2.5" wide and another hole 2" wide. Make a mark on the 6" long side that notes 1.5". It'll end up looking like this: