It's not like there's an international council of underwear classification, and many call them different things. To me, if the legs are pretty short, about halfway between boxer briefs and briefs, they're trunks. I will probably never make a boxer brief as the legs are too long. For trunks, I think a short leg is perfect.
For briefs, I have not found a great one yet where the leg holes stay in place and don't ride up. I will probably make a brief some day once I can get the legs to stay in place, though.
But back to to the trunks. In summary, simplicity was key. It was more important what NOT to do than what to do. Here are some things I avoided.
Labels
A certain "luxury" brand who shall go unnamed dares to sell underwear for over $60, but they leave in itchy sewn-in cloth (satin) tags. The nerve of them to charge so much but overlook the itchy tag is mind boggling.
The Waistband
It's not too narrow so it doesn't dig in or roll. The engraving (embossing) is super minimalist so you're not a walking billboard but it gives it a certain eye-catching texture in the right light.
The Material
It's a natural modal fabric which is made sustainably from plants. Specifically, the bark of birch trees which uses a low amount of water. There is minimal spandex to give it stretch.
The Details
Other brands who I won't name went overboard with trying to put little memos or words of encouragement all over or inside. We don't need to read "Live, Laugh, Love" inside our underwear. It's over-designed and cringe.
The Fly
Now to the most important part. The fly? Once you try a horizontal fly be hooked and you'll wonder why side flies are even a thing. Your body doesn't even move that way! The horizontal fly is really relaxed and can be easily tugged down for relief. And while you're sleeping, you'll notice that you can't really control what goes on down there, and it has a mind of its own. The horizontal fly provides the perfect natural portal if it goes exploring upwards, and it won't wake you up painfully.