Hey Maestrotim,
Are you near Chicago?
I've got the original box.
It measures 22 X 18 1/2 X 18 inches.
There's basically 4 styrofoam inserts. Two large square ones that fill the top and bottom. A hemispherical cap that goes over the bottom and locks into the upper piece. A large thick piece that covers the screen.
The iMac goes in the box with the screen lowered and vertical until the swing arm is horizontal.
The Base is the heaviest part and it fits into a well in the styrofoam bottom part. The hemispherical cap goes over that and pins into the upper piece of styrofoam. It's also formed such that it supports the arm and cradles the screen. A large rectangular piece fits over the screen. The entire arrangement makes a large firm packing crate for the iMac.
If I were packing this I'd try to duplicate this as much as possible. I'd buy a 4X8 sheet of very thick styrofoam insulation and cut it out to duplicate the way the iMac is held in there.
The main thing is that the base must be held securely and kept from moving.
Otherwise if you're close I'll give/send the original to you.
Richard