Andy,
It is built-up (see pic below). As Matt and others have said/shown you should be able to get them to come out around 30-40 grams if you are careful. I wasn't too worried about getting it super-light so mine came in a bit more...I added a bit more material for the hinge post and the counter balance can be made a bit lighter. I lost a few grams after shaping the counter balance and beveling the hinge line, but will put that back plus a bit after I finish the rudder ( Haven't decided yet whether to MonoKote or glass and paint...leaning toward just MonoKoting it). Another thing to do is put the horn above the horiz stab...straighter shot for the pull-pull.
Cutting one from foam should work just as well....it is a fairly straight taper.
-Erik