Maybe this isn't really an INSTRUCTABLE but rather a show and tell. With handy friends and some donated materials, it might cost less than you think....
We moved into a new house and I wanted the cats to be able to enjoy the beautiful Alabama climate but also wanted them to be safe. We had a window in the basement that could be adapted for a "pet door" although it was high and would need boxes in the basement so the cats could reach the windowsill and pet door.
Step 1: Determine Size
To fit between generator and a/c unit I decided on 10' x 14' x 8 ' high. Made list for Home Improvement store for 4x4's, 2x4's, 100' roll of 2x4 fencing. 2 x 10's for framing bottom and used a pre-hung door I found in the basement of home I bought.