Originally Posted by organ
I'd just carve some notches into a wooden dowel.
I used a square dowel from Home Depot/Lowes and used a 1 gallon measuring pitcher to fill both my keggles one gallon at a time. After each gallon settled, I made a notch using a file at the fill line. After I had notched to 15 gallons, I let the dowel dry overnight. I used a Sharpie to write numbers and fill in each notch, then coated the whole thing in cutting board/butcher block oil. A couple days later and good to go. I have a sight glass on my HLT and it is less useful than my "dip stick".