Hi Robert, what I meant by creating a box was to sort of extend the sides of the stove to the wall behind it, and put a partial 'roof' over these two sides. What I would be trying to do is make any noise energy to have to go around at least one or two corners to get from the fan motor to my ear.
Most sound travels in straight lines. Low frequency sounds tend to travel well around corners and through surfaces. This is why we hear the 'boom-box' base sounds so well in the cars with the huge sound systems, but can't hear much of the mid and higher frequency sound.
Most of the fan noise is mid and upper frequency, so you should be able to block it by blocking the direct line from the fan inlet to your ear. This is why I suggested an elbow over the intake opening, with the elbow facing the floor, this would signifiacantly reduce the straight line from the fan to your ear. Since you said that an elbow intake won't work, then try some extended side walls and a sort of roof over the rear of the stove, partially enclosing it. You should be able to keep the clearance to hot metal and around the motors far enough to not restrict cooling.
I can't remember the names of any sound proofing foam sheet, but you can put a layer of fiberglass insulation like for your walls, and use a layer of wire mesh to hold it in place, the fiberglass will provide a sound absorbing surface.
All of the above can be done as a very 'temporary' attempt to see what works, using some plywood or heavy cardboard, just keep several inches between the temporary box and the stove. Try it for a few hours and see if you can tell the difference. If it works, then you can figure out some more perminant fireproof materials. Don't leave the temporary pieces in place if you are leaving the room for any length of time, especially if they are flamable. Safety first!!
Hope this helps, and makes sense, Greg L