To my surprise, the extra panel of insulated glass in the unit was held in by double sided tape, and was easily removed (unbroken and stored if future owners decide to eliminate the dog door.). I came up with a plywood and 2x4 version initially that met the immediate needs, then I ordered some insulated glass from a local glass shop.
This is the third (and final) evolution of the door unit. It is constructed of PVC so it matches the rest of the unit and will last against the weather as well. Total cost was $300.
Step 1: Gather Materials
I was able to find the PVC boards 3/4" x 3-1/2" at Lowes. They were 8' long and about $14 a piece. With my mistakes, five boards turned out to be just about perfect.
You will need:
Dog Door - I used Ideal Pet Products Medium door
Glass for above the door and the remaining frame (See Note Below)
PVC lumber, or Pressure Treated Wood
Screws (I used white headed screws)
Pocket Screws (if you follow this assembly)
Table Saw or Router for Rabbets
Pocket Hole Jig - Even a small Kregg Unit will work
Note on ordering glass. Since the glass will be large and near pets/children, it is a good idea to get Tempered Insulated Glass. It may even be code where you live. The additional cost is not significant, but make sure you ask. When you order Insulated Glass, they will ask you how thick you would like the glass. I used 1/2", there is also 11/16" and a variety of other sizes. They are very accommodating, I believe the total cost of the glass was just over $200 which is well worth it. Single Glazed (one piece not insulated) tempered glass would have been significantly cheaper, but insulated will help somewhat with energy efficiency, but more importantly heat transfer (it is installed in Texas). The glass took just a few days to arrive.
Step 2: Remove the Glass
Behind that, there is a ~1-1/2" pocket just like the pocket the sliding glass door sits in. That is where the new panel will be installed.
Step 3: Rip and Route Boards
When working with insulated glass, you try to frame the internal metal frame of the glass so it is just proud of the frame. That is how I got the 3/8" with my glass. The 1/2" is simply the thickness of the glass.
I constructed the frame with the outside vertical pieces running the nearly full length of the pocket. This meant that I to tilt them to get them into place. I did this to make the frame more structurally sound. Then the top and bottom frame were attached leaving and outer ring with a rabbet all the way around.
If I had been thinking about it (it was over a 100 degrees and raining when I did this yesterday), I would have screwed the unit together outside of its final placement and then routed the rabbet. The way I did it on the table saw meant that the rabbets run the full length of all of the pieces. You can see the gaps between where the pieces come together below. Rather than doing it over, I used longer pocket screws. I reasoned that since the frame would be sandwiched with silicone, enclosed over it's whole perimeter, and had a 6' overhang, it wouldn't affect it much. The gap is only 3/8"...
Step 4: Measure and Install Center Divider and Cross Piece
Then I set the top glass and dog door frame in place to get the vertical width to fill the gap. I had to cut the cross piece three times. As this is the most visible joint, it had to be perfect.
Step 5: Install Glass and Assemble Inside Frame
If you haven't installed glass like this before, start with the smaller piece. I used left over pieces set diagonally where the inside frame would go to brace/hold the glass in place while I started adding silicone to the inside frame pieces. Beginning with the vertical inside frame pieces add a bead of silicone, adjust to get the reveal right, and screw to the outside frame. Next do the same for the bottom rail, then the top rail. Finishing with the center divider, and the small cross piece.
As you screw it together, you should see a small about of squeeze-out from the glass rabbet.
The PVC machines very easily, but I did not sand or roundover the rough cut edge on the vertical center divider. This will need to be done for the inside and outside, and is easier before the glass is installed. All of the other frame pieces have the cut side inside the unit, so the factory rounded edge is showing. The divider between the dog door and glass has the cut side down, but it is hidden by the dog door frame.
Step 6: Install Dog Door and Admire
After about 2 hours, use a razor blade to trim off the excess silicone that has squeezed out.
The dog now has run of the house and backyard, and the dog door looks like it is part of the factory sliding glass door. There are a huge variety of Pet Doors available, some that have electric locks that only open for specific collars, etc. Find the one you want and start there.