I installed a pet door that replaces the glass in one of the sliders and so the door opens and closes and locks normally without narrowing the opening and defeating the gaskets on the door that keep the outdoors, outside. I ordered it from a company called Pet Door Design, and had it shipped here and installed by the company that installed the glass in my house originally. It works great and looks great. Here's the link if you want to investigate, and a picture of my slider.
http://www.petdoordesign.com/
(It seems they're in the process of relocation, but are accepting orders for future delivery according to the website)
