You will have trouble finding a suitable box for double boxing. And that just tends to add on to the cost of shipping these days because the size and girth is larger. I have been going to the local U-Haul store and buying their specialty boxes that are almost 50" long and about a foot square, and then use the material t make a corrugated wrap for the kit box. These boxes are for floor lamps or something similar, may even be for golf clubs, but they only cost about 7 or 8 bucks and if you are careful you can get two wraps out of one box.
You just need a large work table to start with. Also a long straight edge, pencil and a dull pizza cutter. I cut the box lengthwise down the corner where the seam is, then lay it down and the kit on it and bring it up to about half way across the kit box. I trace a line with the pencil, then enhance it with the straight edge. That will be your first fold and where the DULL pizza cutter comes in. Run the pizza wheel down the liune along the straight edge to guide it. That will make a score that will allow you to fold it there. Make that first fold and hold the kit box against the flap and make another line. Again, enhance it and run the cutter down it to make a score for the next fold. Continue this around the box until you get to where you overlap the first edge, and do that by at least 2" and make a line there. That is your basic box. You can then cut and fold the end panels as you desire, or trim the wrap even all the way around, and using some scrap, cut end panels that measure what the new dimensions are and hold them on with tape. Once you make the first one, it gets much easier and kind of like building a model!! You will add some weight to the shipping cost but no added weight from packing material like you would need for an over sized box. and probably save 10 bucks or more due to the reduced girth measurement. I hope this all makes sense , I'm kind of tired!! I have had great success send kits out this way and also receiving them wrapped like this. I think others will agree.
Type at you later,
Dan McEntee