With a well made clay pot, you won't even have to worry about adding hot rocks, etc. You don't want it boiling anyway. Plus, when you take out the cooled rocks to return to the fire, you are removing a portion of the glue stuck on the rock and it burns up in the fire. Arrgh, waste!
Just use a pot with taller sides and place it next to your heat source. Keep turning the pot to heat the sides evenly, stir from time to time, and test with dipping a finger. You want to keep below boiling for the strongest glue after all. The finger dip tells you when it is getting too hot, PLUS as the liquid clings to your finger it allows you to determine when the glue is actually being extracted from the sinew....your fingers will get sticky!
Tell me that technique ain't ABO! But a really true to life ABO would lick the glue off his fingers because it is nutritious and wiping it on your breechcloth wastes it and makes your breechcloth stiff.