I feel your pain.. Westcott makes a whopping 2.2 meter size which seems nice. The tip of the shaft is 7mm and should fit Elinca heads. But at that size, you could use the extra brolly clip in the flash head mount to attach a normal 8mm shaft. For smaller brollies it's nice to have them centered more precisely but at 60" there shouldn't be much difference.
you'll be lucky to find one in 7mm, i ended up getting an 8mm and using the mount on the side of the strobe instead. It would appear that the 7mm ones bend when mounted as its too thin so most folk use 8mm.
M.