In advance: Make cookies. Prepare dough. Press 3 in. long cookies using bar disk. Bake and cool cookies.
Melt white candy and tint blue. Using melted candy in disposable decorating bag, fill in 1 1/4 x 7/8 in. corner rectangle. Tap to settle candy in ridges; add candy if necessary to level surface.
Sprinkle on nonpareils. Using melted red candy, pipe stripes in spritz indentations. Let set.