Recently purchased and installed the pet door with dual-plane glass in my sliding door. The small size still works with 20-pound cats. Initial install was easy, although the content varies a bit between the written instructions and the provided YouTube video. Used the included lock to keep pressure on the sliding glass door against the panel insert, but including an expandable rod would have been nice.
As other reviewers described, additional insulation was needed. I purchased some 3/4" closed cell backer rod foam from Home Depot, then pushed it into the gaps between the panel and the sliding door track. The largest gap was between the two pieces of the sliding door glass since the door is now permanently open. This required 1" closed cell backer foam from Amazon. Overnight temperatures are down to 35F and it is holding up well so far.
Removed the magnets from the flap and have started flap training with the cats. Purina has a helpful "Training a Cat to Use a Cat Flap" guide online.
White / Small / 77.25" - 80.25"