[Steve in SLO]

For those of us who ship bikes, linear weight charges are the norm, so it behooves us to cut down boxes to the minimum safe size to save $$ on shipping. I recently figured out a pretty cool trick for when I needed a box which was shorter in length that I had on hand.
Cut the box off to the Required length
Trim off about 1/2 inch height of the side walls of the shorter portion ( so the top flaps can lay flat) and and insert into the main box.
Take an awl, Phillips screwdriver or similar and poke through the box from the side. I use four sets of holes which are 2-3 inches apart.
Starting on the inside take a large zip tie and pass it through the holes and pull it tight, thereby binding the two pieces of the box together.
Finish with some tape to keep from snagging the free end of the box
Pack it up!

This technique is a lot more secure than trying to do it all with tape, as the box end tends to telescope, thereby loosening up the box.