Line a sheet pan with foil. Combine the salt, pepper, chili powder, and garlic powder. Place the slab of ribs onto the lined sheet pan and cover both sides of the ribs in the spice blend.
Place in the oven meat side up and cook for 5 minutes flip over and cook for another 5.
Remove from the oven and reduce the temperature to 300°F, 150°C, or gas mark 2. While the oven is cooling down spread ½ of the watermelon barbeque sauce over the the ribs. Cover in several layers of foil to prevent the juices from leaking out. Cook for 2 hours.
Once the ribs are cooked brush with the remaining barbeque sauce. Allow to rest 10 minutes before slicing.
Notes
If using a grill wrap the ribs in foil and place in indirect heat. If using a gas grill keep the temperature at 300F or 150C.