Put the iMac into the box it came with, have a carpenter build a plywood box around it and then have it shipped with insurance marked "Fragile"
Be sure to have a copy of your data off the machine too.
Use the original box, packing material, etc. and ship it (insured) ups, fedex, or similar delivery service.
If it's good enough for apple to ship to customers that way it should be good enough for you too.