I had a friend who was quite into muzzleloaders and “rendezvous”s. He would build a firebed, when I asked him he said you follow the tooferate rule-2,4,8. The fire burns for 2 hours,you put 4 inches of dirt on it,the orginal hole you dig to put it in is 8 inches deep. The hole was about a foot wide. He’ld line it with dry stones and build a fire . After it burned down pack dirt on top and then pine needles for insulation. Bingo. I backpacked and climbed with him for years and a health issue led him to the mountain man lifestyle ala driveup…I was always glad he found another passion. Sorry I can’t remember more, try a survival or muzzleloader website.