As far as the boiling side, I am thinking about using black steel/iron pipe 3" X 8' with 45deg upright on the ends to release heat. Or, an upright open angle in the center and both ends. The oil, I was thinking of using lard from WallyWorld.
The clay/dirt (clay soil on my property), I am think of using chimney vent pipe 8" or 10" x 8' to pack the dirt around to green bow. Hang over the fire, with adjustable heights, or bury it on top of coals from a hardwood fire. I'm not sure, but I think that would take a while longer to heat and cool down. I would split the top of that pipe along the top from end to end. I could set the bow in there with the belly down.
Nate