j-d:
Sorry I did not think to take pictures.
The existing Maxxair brackets that I re-used are exactly what you describe. It is an L bracket, with the vertical side mounted to the side of the "throat" of the fan, and then the Maxxair cover was bolted to the horizontal legs.
The Fantastic vent came with flat brackets. You are supposed to remove 4 of the existing vertical screws that hold the bottom plastic flange of the fan housing onto the roof and use new screws in the same holes to fasten this flat bracket horizontally. Then the cover mounts to the flat bracket similar to the horizontal leg of the L bracket.
The downside is that all of this is covered in a 1" wide layer of caulk, which I and others are reluctant to disturb.
