I don't know about the Duluth underwear because I don't own any. I only wear Hanes briefs. But on the clothing side of the issue think about what people wear who live in desert environments. They don't run around naked. They wear something light weight that keeps the sun off but allows air circulation. Nothing is much hotter than direct sun on skin. For outer garments wear something that is loose fitting enough to be comfortable but not loose enough to get you caught in a pto shaft or some other such situation. Also, make sure there is at least one layer of clothing between your skin and the sun. After you sweat enough to get your shirt wet any little breeze that comes along will feel like you are wearing an a/c unit.

Back in the 1970s as a kid hoeing cotton in the June and July sun in Mississippi it was HOT. I usually wore jeans, boots, and a long sleeve shirt. I would sometimes go over to the creek, soak my shirt, wring it out so that it wasn't dripping but just damp, and put it back on. I would be cool (relatively speaking) for hours. A wet shirt works on the same principle as a swamp cooler.

As an aside, I can't imagine how those Civil War soldiers were able to march and fight in wool uniforms in the southern summers.