I would also use a regular manilla file folder cut to size, then make a 'window' for the area available for the signatures. You can make a baseline with permanent market on the inside back of the windowed file folder to help them write their signatures. Have plenty of extra fabric for those who want a redo of their signatures.
When I tried other methods people ignored instructions and wrote in the seam allowances. Thus I developed the windowed file folder system.
I also found white or cream, could be tone on tone, fabric worked best for signature quilts.