Go to Home Depot, Menards, Lowes etc... buy some 2" thick foam (4'x8' is like $10) and you're good to go. I have used it for subs, alternators, batteries, amps, speakers etc... pretty much anything up to 100lbs~ on some of the packages. No issues yet.
Actually just shipped a battery and an alt. this morning using this method. battery has 4" (two layers) all the way around it, alt. only had a single layer but that is more than enough. UPS just requires 2" of packaging on all side for it to be covered by their insurance. Heavier items, I always got 2 layers all the way around though.
For subs... I cut two pieces the same size as the bottom of the box. shove one all the way to the bottom of the box, take the other and cut a hole in the center the same size as the motor, put that on in next. Throw the sub in a garbage bag or something and drop it in the box so the motor fits snuggly into the hole you cut out of the second piece of foam. then cut four pieces for each side of the box and slide them in. cut a coule (as many as you need) pieces to fit inside the basket to get it as close to flush as possible. cut another piece for the top (same size as the very bottom piece) drop it in and you are good.
I had an old POS power acoustik sub I packaged this way. We LITERALLY dropped it off my roof (about 20ft~) and it was just fine. threw it down the stairs and it was still fine. Of course, this was a 30lb~ subwoofer probably, nothing crazy heavy...