Trim the roast of any excess fat, and season with salt and pepper. Heat olive oil in a large skillet over medium-high heat. Place meat in hot skillet, and brown meat quickly on all sides.
Transfer the roast to a slow cooker, and sprinkle onion over meat. Season with chile peppers, chili powder, cayenne pepper, hot pepper sauce, and garlic powder. Add enough water to cover the roast.
Cover, and cook on High for 6 hours. Reduce heat to Low, and continue cooking for 2 to 4 hours, or until meat is totally tender and falls apart.
Reserve 5 cups cooking liquid, discarding onions and peppers.
Place corn husks in a large container and cover with warm water. Allow to soak for 3 hours, until soft and pliable.
Place vegetable shortening and salt in a large mixing bowl. Whip with an electric mixer on high speed until fluffy. Add masa harina and beat at low speed until well mixed. Pour in reserved cooking liquid a little at a time until mixture is the consistency of soft cookie dough.
Drain water from corn husks. One at a time, flatten out each husk, with the narrow end facing you, and spread approximately 2 tablespoons masa mixture onto the top 2/3 of the husk. Spread about 1 tablespoon of meat mixture down the middle of the masa. Roll up the corn husk starting at one of the long sides. Fold the narrow end of the husk onto the rolled tamale and tie with a piece of husk.
Place tamales in a steamer basket. Steam over boiling water for approximately one hour, until masa is firm and holds its shape. Make sure steamer does not run out of water. Serve immediately, allowing each person to unwrap their own tamales. Allow any leftovers (still in husks) to cool, uncovered, in the refrigerator.