The one in your pic would get the job done, but the flap doesn't fold up around the cap for times when you don't need the flap.
Having the flap fold up looks a lot neater when you're back at base and walking around out of the sun.
I think the link I put up showing the dark blue hunter's cap, with the sides able to be folded up, would be ideal in a summer weight offering.
But having said that, the one in your pic does have the advantage of being adjustable, which my old Kenyan cap did not.
I'd certainly wear the one in the pic as opposed to a cap with no side or back of neck protection if I was working in the sun all day. The flap doesn't look as long as you see on a lot of the beach/surf sun caps, which is good.
I expect a couple of velcro tabs would hold the flap up, if the wearer wished.
I much prefer a soft, shapeless crown, as opposed to the stiffened, high crown of most baseball caps.
A soft crown eventually settles into your head shape, and thus is more comfortable. Breaks up the head profile more than a stiff crowned cap also.
Plus it makes washing easier.
I don't know what the brim of my cap was made of, but it got a good soaking numerous times and never lost its shape.
The one in your pic would be ideal in hot desert/jungle environments.
Made in disruptive patterns to match whatever colour shirts and pants, it could be a useful piece of kit.
I'll leave it with you.
Happy to field test a sample one if you get around to designing any.