Measure the height and width of the open window space above the air conditioner with a tape measure.
Purchase the acrylic glass sheet in the size necessary.
Place the acrylic glass sheet in the open window space. Fasten the sheet in the window using a strip of duct tape across the top of the acrylic glass and the window frame.
Caulk around the bottom and sides of the acrylic glass sheet and allow the caulking to dry. Remove the duct tape and caulk around the top of the acrylic sheet. Make sure all the cracks are caulked to prevent air leaks and water damage.