I would just follow their packing instructions .pdf as close as possible. http://www.greatplainsaudio.com/packing_info.pdf
That's what I did, and apparently I did a good enough job they reused my shipping boxes on return. I wouldn't stack the woofers just to save a couple dollars. Just one woofer per box.
Screw the wrapped woofer face down to a piece of plywood and make the top pad with cutout like they describe, cut down the box and that's about it. If you want to go the extra mile I would recommend making corner blocks for the box so if they stack too much weight on it during transit or someone sits on the box, the corner blocks take the weight and not the magnet or spider.













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