After testing a few different options, we found that the best way to add weight to an umoja contact sphere is by injecting water inside it.
You can use a syringe with a 25 gauge needle, so you can measure out exactly how much water you'd like to add - we use a 60cc syringe. We recommend injecting through the thick part of the umoja sphere, near the hole by where the leash goes into the sphere.
You want to push the needle through as much material as you can, so it makes a good seal when you pull it out. You shouldn't need to seal it with anything else. It does take awhile to push the water through the small needle, but a larger needle would cause a leak and would need to be sealed (we haven't tested and can't recommend).
We hope this helps, and if you have any questions please contact us!