Trust me, do
not use a prefab. Learn to make it yourself, and you'll be infinately better and more proud of yourself in the end. I wouldn't use a model either, because then there would either be no clipping (player could walk completely through it) or a bounding box (solid box around the entire model). Models in maps are good for map-model objects (trees, rocks, shrubs, etc), but not major features of the terrain.
Look around and read up on information on triangle brushing. People might have special individual techniques, but they all basically stem from this idea.
http://www.gamedesign.net/node/278
http://halflife.qeradiant.com/design/roe2.htm (this one's for QERadiant, but the principle's the same)
Those should get you on your way. Snarkpit had some good ones as well, but I can't seem to access them.