I would prefer a 13mm hex on the shank so you can use either 13mm or 1/2" wrenches on it.
I was thinking a 16mm (5/8) would be somewhat interchangeable with a S.A.E. wrench. The female ends of my current 3/8-drive extensions are much larger in diameter: 17mm - 18mm. Common Hex Tool sizes are 10, 12, 14, 17, and 19mm, if you wanted to make it dual-use in that respect.
In this picture below, it doesn't look like there's much meat at the corners of the female square-drive, for strength. I realize it's made with clearance in mind, and I'm not an engineer, so what do I know?
If it were made in China, I'd pay $3.00 for one (on a good day).
If it were made in America (from a new start-up company I've never heard of until now, who's warranty will die if said company goes under), I'd pay $5.99 for one.
If it were made in Germany, I'd pay $13.00 for one.
440c stainless is what my cheaper knives are made from - the ones made in China. They're heat-treated as well, and easy to sharpen. I've never wished to put a wrench along the length of an extension - I'd just get a longer or shorter extension from the box. Seems like a solution looking for a problem to me.