Everyone who has a dog around here seems to have one of those doggy door inserts for their sliding glass door. They are very handy since they don't require cutting a hole through a door or wall. Now that I have a fenced area in my front yard, I have been leaving my sliding door open on the weekends for the dogs to wander in and out. But it is getting hot and it isn't very practical to run the air conditioner with the sliding door open. I have seen many doggy door slider inserts for sale on craig's list for $50-$100 but I don't really want to spend the money.
Today I took a piece of paneling scrap and cut it to fit the door then cut out a dog sized hole. I haven't attached a flap yet but the curtain kept it covered for the time being.
The dogs liked it and I liked it because it blocked the hot air. I don't care that I can't see through. It is actually better that the sun is blocked because the door is on the south side of the house and that glass gets very hot in the summer. And it is easily removable if I need to get something big out the door. Best of all, it was free except for the time that it took to trim the paneling.