Janet, buy a chrome rolling shelf unit. You can attach the fan and the ducting to the underside of the top shelf and it will back right up to the window, the added bonus is oodles of more storage too!! The chrome racks roll easy and are heavy duty enough to hold lots of glass.
You can put the shelves any height so it could be made
to fit your system exactly.
You could put a plexiglass panel on the back of it to attach the ducting to and maybe a rubber boot around the whole thing so it would be air tight up against the window to help with heat and cooling.