This is a remix of the excellent https://www.thingiverse.com/thing:6619437 remodelled from scratch in onshape. Design is here in case you want to modify for your own purposes:

Changes

• Flange is round
• 6 holes in flange vs 4
• More material at the thinnest part of the flange
• Minor tweak to dimensions to make the angles round numbers

How to use

• Cut a 7.25 inch (~185mm) hole in the surface you're attaching this to, screw it in. I think I used #8 wood screws. I have a portable Air Conditioner from the local hardware store from a brand I have never heard of and this worked great. The hole size must be quasi standard.