LanParte 1.8lb Counterweight

This weight is made of solid billet stainless steel. Means - a heavier weight packed into a smaller size. So you'll be able to use it for a long time, as there's no chance it'll crack if knocked. It also has a rubber o-ring each side to protect it if damage if dropped. The weight clamps on to your 15mm rails through a single clamp and comes with two bolts, allowing you to screw multiple weights together.

This weight will have a better balancing effect the further you put it away from your shoulder pad, so you might want to extend your rods, to create a longer overhang.
After all - it is the weight distribution that makes the the rig so easy to operate.