I decided to make my own counterweight and it turned out to be pretty straight forward. What you need:

1x 15mm Rod Clamp ($15) (I used one from a Gini Lens support, which already has pre-drilled screw wholes)

1x Project Box (ca. $5) (you can buy them on eBay, Radio Shack, or any other store that carries electronics etc. Preferably it has at least one side made off metal)

1x Weighs (ca. $7) (I got some led for cheap from ebay)

2x Screws ($.20) (I found screws that perfectly fit the pre-drilled wholes of the 15mm Gini rod clamp)

All you have to do, is to drill two wholes trough the project box so you can attach the Rod Clamp and you are pretty much done. Fill the box with as much weight as you need, and make sure to stuff the remaining space with some padding so nothing moves. I used bubble wrap, but you could use paper, foam or anything else that is similar.

As you can see, it is a cheap, simple, yet flexible solution and even looks decent...