Besides, I found a good enough deal that it didn't cost that much more to buy it.
I bought mine from the Chickencoopdoor. The only US provider I found for $150.
Here are directions if you want to do it yourself. The second link says he paid $120 for parts. When I saw that, I decided $150 wasn't bad.
Dinousaurs and Robots Chicken Door and BuildEazy Chicken Door