Installing air conditioner for sliding window?

If all else fails, cut a piece of plywood and duct tape it on?
Might look a bit ghetto, but probably as good as anything you can buy, functionally. Window a/c are pretty ghetto to begin with.

The first time I ran across this problem I cut a piece of thick cardboard** and used it to fill the gap with some duct tape to hold it. Make sure it's held well because on a really windy day it can get sucked right out. So that it wouldn't look like ugly cardboard I spray painted the outside to roughly match the building's exterior color. I think I did something similar to the inside surface. A month or so later when I got around to it I got some plywood and cut that to size, again painting the inside/outside to look good. I screwed it into place so that I didn't have to use the duct tape which doesn't stick to plywood very well anyways.

**You can glue a couple or more sheets together to make the cardboard thicker and stiffer.