Window too big for a/c

I just went to PC Richards and then to Home Depot to see if they had a simple solution but they really had nothing more than the same improvisational ideas I could come up with myself. I have a really tiny room with a big ass window (width=41"). I just need a small 5000 BTU a/c unit but they all max out at about 36" width-wise.

What would be the best way to fill the gap?

We handled a similar problem with a piece of rigid insulation just set into one side of the window and cut to height and width.

If you center it, you only need to cover 2 and a half inches on each side.
Not a big deal. As suggested by OLDGUY, You can use rigid insulation, plywood, or pieces of wall panaling. Run a bead of pliable putty…the kind that stays soft…and press it into place.

Most/all window A/Cs come with expanding side panels of metal and pleated plastic designed to be pulled out and screwed to the sides of the window frame/return to fill the gaps on each side of the centered unit.
These panels work reasonably well, but do not provide much security. If the unit is accessible from outside make sure it and the side panels are securely attached to the window frame/sill. Even then it is probably a weak point in the security of your home.

Kinda depends if this is a one time install or to be seasonally repeated. If the latter, then I’d go to the extent of fabricating a frame from 2 x, rabbet it to fit nicely on the sill and accept the upper sash, integrate compression seal strips into the design, and prime/finish paint it to match the window assembly.

[handily declined person] What is this “pliable putty” you speak of. If I go into Home Depot and ask for “pliable putty” will they know what I’m talking about?

Nah, just a rental, and a month-to-month rental at that. Not sure where I’ll be next summer.

It’s made by ‘FROST KING’.
Called ‘fingertip caulk’.
Comes in a roll of 90 ft. x 3/16 inches.
You unroll the length you need, and press it in place. It will come off in one piece. It stays pliable…no mess.

The most widely known name (in NE US) for the pliable putty is ‘Mortite’.