Thanks for the (mostly) kind words.
Here is the Aluminum channel I started with:
6061-Aluminum-Assoc-Channel-15-x-2-25-x-5-x-48
(If you check the purchase history, I'm the only one who has bought one - back in December, when I started planning this project)
And here are the wheel extenders I'm using:
Pro-Line-23mm-Hex-Adapters