Look this is not spectacular engineering or anything.  Have had a few encounters with stuff getting into the fan blades.  Plus a few close calls with the cats who despite the danger insist on going up there anyway.  Did not find any online to order so that means garage engineering time.  Took some scrap 3/4 plywood and made a frame.  Cut the outside circle with my bandsaw jig then used my router to do the inside.  That got the back piece made.  Have several pieces of this wire shelving people toss away.  Took that and cut out a circle then laid it on top of my wooden circle.  Marked where the wire crossed over the frame and routed out for the wire.  Once done I put a piece of scrap 1/4 paneling screwed it fast and routed the edges flush.  Added some yellow paint for safety.    Then took to the fan itself and added two brackets to the fan shroud and some screws.  

 

Only hold up was the wire for the fan motor.  It has always been over the outside of the shroud.  For this that would be a problem.  So I drilled a hole and used a old tire valve stem.  Inserted it as my grommet and ran the wires through that.  Now the wire is in the shroud, and protected. 

 

Simple but effective.  Going to make one for the house attic fan as well, once it cools down some.   

 

Clever, the use of the shelving for the grate worked well.