bike boxes are flat, folded, rectanguloid sections of compressed paper.
interestingly, refrigerator boxes have the same definition, if not the same dimensions.
same is true for postal boxes, fruit boxes, etc.
cheap box-cutter, roll of duck tape, measuring doo-dad....done.
often less time-consuming to grab a bunch of cardboard throwaways
and patch together your own instead of calling/visiting bike stores.
easier to get a bike to fit in your custom box than midget bike store boxes.
remember to print off the airline sporting goods/oversize baggage size limits.
or.....just take an empty box!