Many people have inexpensive air conditioners that are intended to fit into a double-hung (vertically sliding sash) windows and want to use them in horizontally sliding windows. If you already have some of these air conditioners or are attracted to them because their price tags are significantly lower than that of casement units, portable units, or central air, it is possible to make a frame to adapt them to sliding windows.
These solid, simple frames require no exterior brace. This is perfect for apartment dwellers and those who only need an air conditioner a few months of the year. It takes about an hour and some basic lumber and tools to construct these frames. Once built, your air conditioner can be installed and uninstalled in minutes. (Please note that window air conditioning units aren't intended to be permanently installed. These frames also make it simpler to remove them at the end of the warm season.)
The units pictured include unfinished plywood. However, this can be finished, painted or replaced with plexiglass or other materials, as the sheet lumber simply covers an opening and is not structural.
2X4s *(2 or less but the amount varies)
12 deck screws (or other long wood screws)
These window sills in the picture are fairly deep, allowing for the use of 2X4s. You may be able to substitute other lumber for shallower sills.