I have one and it's great!
I looked at the plans to build one and Brewer and I decided that constructing one would certainly cut into rare and valuable bike riding time, so we opted to buy one. He also has fond Boy Scout memories of heating hot dogs up to about 95 degrees on a 110 degree day in shoe boxes lined with foil.

The one we got is called the Global Sun Oven, available in a dozen places online, we found one locally and the seller matched the price. It's great! It's hot, at least if the sun is shining. It's still ok for stew or casserole if there are a few clouds, but the temp drops too low for baking when it's more than a little cloudy.

Our only complaint was that it took a few days of sun-baking for the painted lining to cure and quit releasing black clouds of paint fumes. This was also fine for food cooked inside a pot with a lid, but potatoes baked just in their skins tasted horribly of paint. Now that we're past that we've no complaints. I'm sure we'll use it every day in July and August when it gets too hot to breathe.

My parents live in Mexico, and we'll be taking one to them in fall when we visit.