Combine the shrimp, crabmeat, mayonnaise, celery, scallions, lemon juice, dill, and crab boil seasoning in a bowl.
Melt the butter in a large skillet or griddle over medium heat. Add the bread and rotate and flip it to coat both sides with butter. Toast in the skillet or griddle until lightly browned, 2 to 3 minutes per side.
Spoon the mixture in a column down the center of the bread, adding the lettuce leaves, if desired. Fold the bread around the filling as for a hot dog on a bun. Serve immediately.
Tip: Look for canned pasteurized crabmeat in the refrigerated seafood case at your market.