Where can I get a bracket to mount an AC at the top of a double-hung window?

The best solution would be an angle bracket screwed into the outside wall below the window to provide the support you need. That's obviously the safest. You'll get additional strength to keep it from sliding out the window if you remove the plastic accordion spacers from the right and left sides.

You don't need them anymore and closing the window directly against the case will provide better support. Another solution is possible if the air conditioner can be slid in and out of its metal shroud. Put the empty shroud in the window and screw it to the window frame on the side and at the bottom.

Then insert the air conditioning unit back into the shroud. Measure the amount of open space between the top of the airconditioner and the top of the window. Run to the hardware store and buy a sheet of 1/8 or 1/4 plexiglass cut to fit snugly.

Slide it in and seal any cracks with tape. Edit: If you can't mount anything outside, how about an L-shaped bracket that's attached to the inside on the ... more.

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