Mounting a bypass humidifier in a small space

[JohnBauder]

I live in a condo with a very small furnace closet. The only duct where I can mount my new humidifier (Aprilaire 500M) is just 11" wide. The plenum hole needs to be 9.5" X 9.5" so that's OK, but the outside dimensions are larger that 11". I was told I can build a mount for it but I can't picture what I need. Could someone please describe this process in detail?

 

[Reach4]

I just looked at mine. I think the mounting screws are about 10.5 inches apart. It seems to me that you could just center the unit, and have the left and right of the housing stick over a bit. The screws would screw into the existing metal.

I was going to use flex, but I ended up using two of these:
https://www.menards.com/main/heating-cooling/ductwork/ductwork-fittings/adjustable-elbow-28-gauge-duct-fitting/p-1444432230271-c-14260.htm?tid=-3544412958795200175&ipos=21

I could have used one of these into the return duct, except I had something else to use: https://www.menards.com/main/heatin...uct-board-fitting/p-1444432197545-c-14260.htm

I used an adjustable piece in the middle. It might have been this https://www.menards.com/main/heatin...olts-duct-fitting/p-1444432200999-c-14260.htm but maybe something else. I covered the joints with aluminum tape and I used furnace cement https://www.menards.com/main/heatin...d-fireplace-mortar/p-1444439437163-c-6845.htm on some old holes. Looks professional-- at least much better than flex.