Well, I have a set of Honjo 43mm fluted fenders I just installed on my Mercian. I thought they felt pretty solid, but the only thing I can compare them to is SKS chromoplastics, which felt a little flimsy to me.
I think I am going to take one off the Mercian and see if it will work with the Shimano R600 calipers. If so, then I will go with the VO 45mm, if not, I will hold out for the 37mm.

Thanks everyone.