Engineering would be impossible. There pretty much actually has to be a small lightsaber there is the catch. If you don't have that, there might not be enough room for longsword lightsaber with the attempt to make it flush with the hilt. My bet is the fight one of the things is at least one side of the crossguard might find itself cut off much to the bad guy's chagrin. There's always pluses and minuses in any design.