I found this AC cover on Amazon for a fraction of the cost of a custom cover. It comes with buckled straps to prevent wind from getting under it or blowing it off. One word of warning through, this cover is only for free hanging window ACs. If your air conditioner has support brackets attached to the bottom, this cover may not work. Although I have brackets on mine, they are not attached to the AC unit itself so I can lift the air conditioner just enough to slip this cover over it and then allow it to rest back on the brackets. Works at keeping the cold and rain out, protects the AC, and looks good.

Been in use for almost two months. So far its been through three snow storms (one a nor'easter), multiple downpours, ice storms and many, many days of temperatures in the double and single digits. I was concerned about the straps being able to hold the cover on after reading a few reviews but they have worked with out fail. Unlike the product photo I made sure the strap closest to the wall is about 1" to the edge of the cover to make sure the strong winds we get here would not get under and lift it.

This AC cover has performed better than any I've used in the previous 20+ years. I thought I had ordered the cover with the fleece backing and was rather disappointed to find this was just a rather thin waterproof fabric. I'd had a nice one with a thick fleece backing that covered 3 sides. It worked very well for many years. I threw it away because there were spiders on it (yes, I did LOL). I could never find that kind again so I've been using the silver or fabric ones with flimsy interlocking straps. All of them flew away with the first good storm until I started putting a piece of marble block on top. That still wasn't enough because as the harsh winters progressed, the slippery cloth worked its way off and the straps were nowhere to be found. This cover is fully waterproof and has faced hurricane winds without budging. I have an inside quilted cover but I didn't need it because no air gets past this Jeacent cover. The pair of straps fit nice and tight without sliding. It is looks dry and like brand new every time I inspect it. I have an extra large window unit and this fits perfectly. It looks like a beautiful box with no wrinkles. I highly recommend this cover. It looks like it'll last a lifetime.

Dimensions for large cover are correct, my unit is 1/2 inch smaller in height, width, depth, so the fit is PERFECT. I especially like the two straps that come with the cover. Each strap is actually two straps connected with an elastic band, this makes creating a tight fit easy keeps the strap tension from damaging the unit. Be careful when putting the cover on and taking off, they have plastic seams which are never very strong.

It's a little big for my small ac, but it resulted extra fabric to tuck in the crevices around the ac, so that is a bonus.
The straps are awesome and has proved helpful and remained tight and slip resistant during a 65mph wind and rain storm