There's always a thread on this topic in this forum.

The OP can get a 32-spoke wheel with lightweight road rims or a minimal spoke wheel like one of the Mavics.

If I wanted a second wheelset for my cross bike, I'd probably just get some Aksiums. Should be more aerodynamic than some OP/Ultegras which I actually do have on my road bike and while going downhill, they sure are noisy. Maybe not the most technical measure of aerodynamic resistance but it seems plain that they don't slice through the air very well.