I believe when I sent my 2010 27" iMac to OWC for a SSD upgrade, they sent me a heavy outer box for shipping, to slightly more than double the thickness of the original box. It apparently worked well, since I had to send it back a second time for a warranty issue, and my iMac continued to work well after that.
It's also not hard to construct an outer box to fit. Making it a bit larger for allow for styrofoam peanuts, or even air pillows, would make sense.