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I wanted a way to let my cat onto our porch, without letting all the cold air in. He loves to sun on the porch, and I wanted to move his litter box out there to free up the closet it had been in. I looked at pet doors a few places, but was not prepared to spend that much. My door is lacking some of the features of the doors I looked at, but they were features I didn't need, and mine was essentially free(since I already had the duct tape). I wanted it to be a two way door, so I didn't need features that let me set it to one way. My porch is enclosed so I don't need to worry about locking it when I am away or to keeping other animals out. The porch is not heated, or even insulated for that matter, so we just needed something to keep the draft out.
I used:
1 8.5x11 sheet of paper
Duct tape
Step 1Make sure the opening is pet safe.
In my case the door to our porch had a broken pane of glass which had been covered with a very classy piece of cardboard for a year. Making the hole pet safe, for me, involved pulling shards of glass from around the edges with pliers.