Why not skip building something, and wrap the pot in an old sheet of some sort, and stuff it underneath the quilt/sleepingbag etc. instead?
Remember though folks. Cooking is started on regular heat-source. The hay-box is only used to "finish it off" or keep it warm.
I have cooked numerous chickens and beef and stuff in my sleepingbag, during a hike. The dinner was ready, as soon as i have set up camp for the night. Works A+
Few good sized stones in campfire during morning-coffeebrewing, prepare the meat with a bit of filling/spices etc. Wrap meat around the stone/stuff stone inside the fowl, wrap meat/fowl with tin-foil or equal, bit of newspaper to soak up the wet stuff, and a bag of somesort to prevent it dripping out into your sleepingbag.
Put sleepingbag in your rucksack, and start hiking.
When youre good to set up camp for the night, the fowl/meat is ready to eat, and steaming hot.
Word of advice: Do NOT try to cook "dry" stuff, e.g. potatoes or the like with this method. It will most likely cause your sleepingbag to turn into either melted plastic or charcloth.