** there is a twist. He wants to install it at the TOP of a double-hung window opening. **
While that type of mounting should be ideal to allow it to cool the best (hot air being at the top of a room and all) while not blocking a view, all window air conditions I've seen rest their weight on the window sill. Since there is no sill at the top of a double hung window to support that weight, you'd have to construct something (if the landlord will allow you) as I've never seen any mounting 'kit' designed for that purpose.
You can find links to some standard window air conditioner mounting bracket supports at the following link:
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