Cut 3 (30-inch-long) sheets of foil. Fold 1 sheet lengthwise in half twice; repeat with second sheet. Fold remaining sheet to fit inside back edge of slow cooker; place in slow cooker. Crisscross foil strips; use to line bottom and sides of slow cooker. Spray inside of slow cooker with cooking spray. Add egg mixture. If necessary, press toast cubes into egg mixture with back of spoon to completely moisten toast cubes.