Step 1: Low Tech, But Effective
I built this dog gate from left over scrap lumber but all you really need are a few 2 x4's that you can rip on your table saw plus piece of 1/8" or 1/4" thick hardboard.
The size of the gate, both the width and height are variable depending on the size of your dog AND the size of your door. For that reason I am not recommending specific dimensions.
The next few steps will detail the basic design which you can follow or modify as needed.
Step 2: There Are Four Major Components to This Gate
1) A base, ripped from your 2x4. My base was 1 1/2" tall, 2" wide and approximately 7' long
2) Two 1/2" square strips nailed to the base form a track that the hardboard gate slides in. The length should be at least as wide as the door opening.
3) A left and right gate support to hold the gate in place when open or closed. The supports are "U" shaped and fit over the outside of the base. Use two screws to secure each support to the base.
4) An additional support (not shown in this photo) which helps to stabilize the base. The design and location of this support will vary depending on your situation.
Step 3: Gate Base and Track
Note the heavy door mat used to stabilize the gate base. The dog gate is very light weight and therefore isn't very well balanced. By pushing the mat against the base it presses it to the door sill which helps to steady it.
Step 4: Left and Right Gate Supports
There is not much to be said here except that the left support has a screw near the top that is used as a stop for the hardboard gate.
Step 5: Hardboard Gate
I screwed an inexpensive door handle to the gate to make opening and closing easier.
Step 6: Secondary Support
There are probably lots of ways to add support to the gate. A few ideas that come to mind are flipping the secondary support around and simply using a small piece of Velcro to stick it to the door frame. Wider feet at the ends of the base would be another option. Since you use Instructables, I'm sure you've got a solution rattling around in your brain if this is a problem you encounter.
Step 7: It's Never Too Late
One day I'll get around to fixing all the scratch marks on the door - I'm just waiting for someone to post a helpful instuctable!