Most inexpensive A/C units are designed for vertical sliding windows, which creates a problem if you have horizontal sliding windows. Installing an A/C unit inside of a horizontally sliding window does not have to look bad, and it does not require any special brace.
I can't take responsibility for your installation, theft, break ins, and your A/C falling out of your window.
Step 1: Materials Required:
The unit I am working with is a Haier 8000 BTU from Walmart. http://www.walmart.com/catalog/product.do?product_id=5402964. Yours might be different but the idea is the same.
1/4" Rubber Foam Weatherstrip
Window screen frame
Plexiglass - Home Depot
Screws, self drilling, 1/4"
C-channel Aluminum pieces 3/8' and 1/2' sold in 8' length
Silicone - Home Depot
Nut Rod Coupling and a screw that fits.
Step 2: Modification
Sorry for the lack of pictures, I had already installed my unit, and than I was asked to do an instructable on it.
The main point of this step is to build a frame around the A/C unit that will fit inside of your window frame. If you chose to use the plastic that came with the A/C follow these steps. However I can't take responsibility for your installation, theft, break ins, and your A/C falling out of your window.
Do not forget to make the 5 degree tilt on the frame for proper drainage.
You could also use the C-channel or square tubing (both available at Home Depot) to build a frame around the A/C so it will fit inside of your window frame.
In the installation manual the first thing they ask you to do is to mount the accordion looking curtains.you should throw them away.
The inside of the plastic is slightly rounded on the inside of all corners, use a saw, or cutters to give it a nice 90 degree angle.
Before you mount your plastic around the A/C, break off the metal on the bottom of unit, or bend it out of the way. It's not needed, it will not fit in to the window rail.
Your frame has to be well attached to the A/C. Use more screws if in doubt.
Step 3: Installation
Measure your A/C from left to right including your frame. Cut two pieces of the C-channel in both sizes1/2" and 3/8". In the 3/8" C-channel insert nut rod coupling on the end. Drill hole on the side, through channel and coupling, than insert a screw. Make sure it's flush on both ends, since 1/2" channel has to fit over it. Refer to pictures.
Measure your window from top to bottom. When you measure, make sure you measure inside of the frame. Your C-channel has to fit inside of the frame. It's very important, since this is going to be holding part of the weight. Cut both C-channels 1/2" and 3/8" to the measurement you got.
Place your A/C inside of your window frame and slide it in to the side frame as far as you can. If the frame around A/C is well attached, you shouldn't worry about it falling out.
Insert the vertical 1/2" C-channel, and drill a hole right about where your horizontal channel with the nut coupling meets. Screw the two together. Also, at this point you might want to screw your C-channel to the frame around the A/C unit.
Insert the 3/8" C-channel inside of the 1/2" and close the window. This will push them together, allowing you to see where the gaps might be. Screw your C-channel in place using self-drilling screws. You will have to drill through window frame. See pics below.
Step 4: Plexiglass
Now that your A/C is in place, and the window closes, I bet you got a smile on your face. However, you got this big hole right above the A/C. That's where the plexiglass goes.
The way this works, plexiglass is inserted into the rail that the screen would have gone in to.
You might want to start with measuring out sizes for your screen frame. Once you've done that, start by putting together your screen frame. Make sure it fits in to your opening, very important before you cut and glue your plexiglass.
Measure inside of the frame, and cut your plexiglass to size.
Inject the silicone in to the grove around the screen frame, then place plexiglass. You could use masking tape to hold the plexiglass in place, while the silicone is drying.
Plexiglass window is ready to be inserted in to place. All you need is 3/8" C-channel to keep it in place.
Notch the very top and bottom, so it fits inside of the of the screen rail in the window frame. Use some more screws to secure it.
Apply 1/4" Rubber Foam Weatherstrip anywhere you see fit. Cover all gaps where light shines through.