Use a wooden corner molding with a notch cut near one end for the single edge razor blade and a window lock (thumbscrew driven clamping device - approx. 99 cents) to position the tube end. I've duplicated the CMR tool almost a dozen times for myself and others. You can even glue a "ruler" to it. I just made pen marks at my favorite length locations and then the thumbscrew makes a nice dent to find those spots easily in the future.

From a post by FS in rec.models.rockets