Create your own tri stock or reduce the size of store bought stock. Simply cut a square notch the size of your stock into a 3/4 inch block of oak or pine.  Glue another block on top of the cut out as shown.  With a thin kerf  hand saw cut a notch for a razor blade to slide in place.  Carefully slide your square stock or tri stock thru the tool.  Choose stock that is on the soft side for best results.  Image shown is for 1/4 inch square or tri stock.