Alex, the best way to cook a pork belly is Sous Vide. You can cook the pork belly until tender and then slice off pieces to crisp and serve. Nothing you cook till crisp today will still be crispy after being refrigerated. You would beed to process it back onto a grill or frypan to get crisp again.........Good Luck........ChefBillyB
As Billy said, it wont stay crispy after it is cooled. However that is perfectly fine! 95% of the pork belly you buy in restaurants is prepared that way. Simply cook the pork belly as desired (braise, roast, sous vide etc..) and then chill/refrigerate. Once cooled, cut your belly into the desired serving size. To reheat/crisp, you'll want to sear/brown the pieces in a skillet/pan/flat top etc.. if the portion sizes are larger, you can finish them in the oven after the sear/browning step.
Pork belly cooking is slow process if u want good results Oder a skin on boneless pork belly with the sharpest small knife just long cuts not to deep rub salt on skin 140 degrees 3 to 4 hours on resting rack deep try with some water or master stock underneath after cooking when skin is dry out and it it some nice colour on press it with some weight or four hours or longer cut it according to portion size . To cook start it on pan with some fleaky sea salt and oil finish it in oven.