Hi there,
I use CC3, DD3, and City Designer 2 for my Glorantha stuff. Most of my stuff is in Dragon Pass, but I do have a "master" file covering Dragon Pass and Prax which I could send you. It's pretty much empty of all the social stuff - it just contains the contours and watercourses of Prax, so you should be able to add stuff to your heart's content! It is Third Age, however; AFAIK there weren't massive changes to coastline, water levels, etc, in Prax, so it should be fine for Second Age use.
Drop me a pm with your email address and I'll send you the file if you're interested.
Happy Mapping!
Sarah