this is pretty cool... i'll probably put one together at sometime.
i just thought of one potential improvement... in price for sure, and maybe even looks.
as for the ruler, you could try one of these printable ones:
first, make sure you follow the printing instructions (see the note at the top of the above website)... cut it out precisely, and tape is down with some good packing tape... i'm sure it would look real slick as long as you do a careful tape job.
if you really want to get serious... you can also try to edit these (by hand or electronically). you could try to relabel/rescale them so that the "0" lies at the 13.5 inch mark from the dowel. then it will give a *slightly* more "dummy proof" reading of HL/HH...