I must admit to having spent a good deal of time searching for something better. I have even looked at the rear wiper blades of some small cars. I can't find anything that uses blades as short as those used on a Morgan.
I have a suspicion that the blades on my car (that will take a standard rubber refill cut down to size) are actually from the headlamp wash-wipe of an early-80s Volvo 245 - IIRC this was one of my earliest contributions to Morganeering when the inadequacy of the OE wiper blades was first remarked on by my father, and we happened to have said Volvo parked next to the Mog in the driveway. A bodge was required for the attachment to the arm, but it has stood the test of time.