I kind of remember reading someplace about using a hot plate, metal vent ducting and a metal pan with a flat bottom.
The metal duct needs to be wide enough to fit the stock with the length, plus one foot extra, in length.
Hang the stock in the duct three inched from the top. Cover with foil or another metal pan.
Place the pan on the hot plate, put the duct in the pan.
Turn it on and check often.
The oil will start to run/drip off the stock.
There's probably more to it. I don't remember where I read it.
Sounds like it would work.
Probably work just as well, if left in the sun without the hotplate.
I wonder if you boil a pot of water on the bottom, would the steam remove the oil/grease????