Hi Art.
So do you have a new iMac Retina without a removable stand, or one of the older models with a removable stand?
if the former, get one of those huge suitcases you can often find at Thrift stores for around $10 (most people don't like keeping them around, because they are so big), but make sure that the width is about equal to the height of the Retina iMac plus maybe 2-3", and then get a few big pieces or a roll of thick foam (the one that cheap mattresses or mattress toppers are made of), put down a layer (mine was about 3-4" thick) with enough area to reach up the sides of the inner area of the suitcase (to serve as cushioning for the edges of the iMac) into the suitcase first, then lay the iMac, face down, into this foam bed (i also spread out a towel to shield the iMac from foam dust, just in case) and then cut another piece of foam to cover the entire back of the iMac...maybe even double it up for extra protection...basically pack as much foam below and above the iMac as your suitcase will hold...i always travel with one of those suitcase handle-scales, to avoid bad surprises at check-in, to make sure that i am within a pound or two of the max allowed for a piece of luggage...i think the iMac plus suitcase falls within the 30lb limit, if you find a suitcase that is fairly light (and those huge suitcases usually are)...since this is not for the faint of heart, you may try to find a thin piece of light mylar or polyurethane to place at the very bottom, between the foam and towel, to further protect the screen, but i've omitted that part during my last two trips and was fine...still a bigger risk than most people may wanna take with a piece of equipment this expensive.
if you *can* take your stand off (as with my 2009 iMac 27"), i found the biggest size hard-shell Samsonite (or whichever brand you can find) to serve as a great transport case for flights...i find those Samsonites routinely for around $10-15 as well...as far as the cushioning goes, same principle: pack as much foam around it as you can.
hope this helps.