I want it for measuring sparge water... A dip stick would be fine. Good idea! Why does it have to be outside your kettle? One of those would be easy to move from kettle to kettle; it could serve the place of 3 sight gauges, right?
A piece of stainless steel, weighted at the bottom and calibrated at 1/2 gallon intervals... There's a certain amount of bling in that fabrication, too.