Do you have a favorite Xacto type knife you use? I'd like one with a larger handle as I have arthur-itis and holding small things can be a chore. There are so many out there I was hoping someone could narrow the choices down.
It's probably a little thin to help Arthur with his itis, but the lower of these two is my current go-to (the upper was made as a gift).
Like Carl's appears to be, these were made with hardware from @PersonMakeObject . The only potential drawback, depending on your tastes, is that they're only designed to take the larger blades designed for the big X-acto #2 handles (and/or Swann-Morton blades, but those are a little fancy for me), which aren't quite as common (or as cheap) as the smaller blades.
I would be tempted to get one of the sets like Vic says and then make a bigger overhandle out of wood and glue the aluminium handle inside it. You can make the overhandle as large as you want. I would,suggests making it on twin offset centres to produce an oval handle as it feels nicer in the hand , as well as not rolling as was mentioned earlier in this thread.