Check out CabinMax
I've come a little late to this discussion, and my travel is largely for leisure.
I came across CabinMax a while ago - a UK company (based not far from Mr Mac, I think) which make luggage to the absolute maximum size allowed by different airlines as carry on.
My current backpack isn't pretty, but takes up to 44L, and hasn't even been questioned, never mind refused by any of the airlines I have used (including EZY and FR).
I have no connection with the company except as a satisfied customer.
I much prefer a backpack, because it gives me an extra hand for passport, boarding card etc etc, and the liberating experience of not going to check in, not worrying about a lost bag and not visiting the carousel is well worth the extra effort planning my packing.