I have heard that the Byron rotor makes a decent pusher prop, although I have never tried it.
With the fan housing and a short thrust tube (~ 8") it works very well on my Enforcer. You can always try the Byron rotor and if it doesn't work out buy a pusher prop. I'd be concerned about the hub strength on a pusher prop if you are going to be turning it at 20K. If you go that way I'd recommend buying a large pusher prop and cutting it down to the desired diameter.

Joe