I highly recommend double boxing the unit. Try to find a box somewhere, like perhaps from behind Best Buy or similar. Then grab a couple other boxes so you can cut them and line the main box with a double layer. Maybe even triple layer. Use a box cutter or razor blade, cut the boxes so you can slip inside the main box on all 6 sides. We ship aerospace parts this way and it works great. Your new used box will be super strong.
Good luck, John
Good luck, John