Combine salt, onion powder, soy sauce, and packet seasoning into a medium bowl and mix.
Add belly to the bowl, evenly distribute seasoning, then marinate for 1-24 hours, covered. Longer will be better.
Remove belly from the bowl, roll the belly up and tie so it holds a shape while cooking.
Using a pot only slightly bigger than the pork bellies, add the rolled pork bellies to the pot then fill with water to reach 2/3 up the larger belly and bring to a boil.
Lower the heat to maintain a medium boil, and rotate the pork 90 degrees every 15 minutes.
The pork is ready when the skin is soft and easily pierced by a chopstick, but the meat inside remains firm, about 90 minutes of boiling total depending on heat level. Remove from the water to cool, then remove twine and slice.