I would not use anything against the driver cones just use plastic wrap. One-inch Styrofoam should be sufficient but 2" better, 42lbs. is not that heavy. I've not used the shipping company you've specified but FedEx has been better than UPS for me.
2 specific questions about shipping 42 lb speakers within US
I've read a lot of good advice. I'm shipping some equipment and the hardest are my old B&W DM-603 S2 speakers. I'll be building a box using 1' or 2' foam boards. I have two specific questions and would appreciate any input.
Q1. Should I use something to prevent the speaker cones from moving? For example, by placing an air pillow or bubble (like the kind Amazon uses), then applying the plastic wrap around it. So the cone should not move during transit. I'm considering that because I've read that the speaker cones can move a lot and the magnets could get dislocated.
Q2. Has anyone used CoolParcel.com to book the shipping?
It seems to be like a priceline for shipping. I input my package weight and dimensions, then CoolParcel shows me a bunch of quotes from Fedex, UPS, and USPS. Fedex Ground is cheapest and about 30% less than my quote from the Fedex website itself. Perhaps it gets a corporate rate and volume discounts that I don't get it? I'm inclined to give them a shot but would be great to hear if anyone has used them.
Thanks in advance.