Those SNP's are way too leaky to get a 12hr cook from a load. Easy Peasy in the UDS.
Ya gotta have control over the airflow to regulate the consumption of fuel. With air leaking into the firebox through all the door gaps you have little control over the temp except for how much charcoal you put in.
You can IMPROVE your situation... use a "maze" style charcoal box, seal the airgaps (maybe temporary foil tape, magnetic sheeting, adding flanges to the doors to seal better), insulation, lower the exhaust to the grate level, add a heat deflector above the firebox hole (reduce the hot spot), add tuning plates (evens out the temp across the cooking chamber)
Build up that UDS and sell the SNP on CL. (probably cheaper too !)