I also have a Topeak Beam Rack, and I also think it kinda sucks. If your ride is real casual it's ok, but it likes to swivel around a bit. All racks that only attach to the seatpost without a secondary attachment point do.

Here's someone with a rack on their Surly Pacer -
yakbicycle: What I'm riding now...

There are a number of racks designed for bikes without rack mounts - they attach at the top under the rear brake bridge, and at the bottom through the quick release. Both these points are designed to handle heavy forces - the brake bridge handles the weight of the bike stopping including the rider (so 200lbs or more), the wheel quick release is designed to handle the entire weight of the rider. As people have said, you can probably attach at the bottom where the fenders mount as well.

If you want to put a bag on the top, the Axiom Streamliner Disc DLX is probably the cheapest -
Streamliner Disc DLX « Streamliner Series « Racks « Products « Axiom Performance Gear

The Bontrager Backrack Lightweight is one of the lightest -
Bontrager: BackRack Lightweight (Model #08214)

Here's it's on a Trek Domane full carbon race bike with no rack mounts whatsoever -
**********-ITEM-?Y's Road ???

There are other options as well, Tubus has an adapter kit for all it's racks to let you do the same thing. In my experience, all of these choices would be far better than the Topeak beam rack.