Luckily enough the Zhumell Dob OTA material is a plain steel which is naturally ferrous. The solution came in a snap. An old steel (also ferrous) counterweight scavenged from my old cheap refractor mount (4.2 lbs), a couple of strong neodymium magnets harvested from a broken hard drive, and a stripe of blue masking tape for scratches prevention - all were sticking to each other almost by themselves.
The counterweight could be placed anywhere on the OTA now, and the perfect balance is just a simple push or pull away. Best locations for the counterweight are apparently on the opposite side from the focuser. Considering that it would be better to chose a counterweight shape which will go freely between OTA and the right fork in such positions. However it works OK on the top of the OTA almost as well, except for the close to the zenith positions. But around there you usually don't need a counterweight at all (NOTE: It is possible that the weight might add some shake resistance in near to zenith positions, need to verify that)
If there is no suitable heavy chunk of ferrous metal at hands, you can get a bunch of smaller pieces and put them into a suitably sized freezer bag (zip lock) along with magnets stuck by a tape to the inside of the bag. Tried that with lead shot - works very well if not better too. Just make sure the bag is sturdy enough (or reinforce it with some more tape), because those neodymium magnets are incredibly strong and may tear the plastic.
- The screw in the side is a tactile indicator of the magnetic line which magnets are forming to increase the contacting surface with the convex surface of the OTA.
- The masking tape also goes to the side of the puck, creating a different texture so you can tell where is the magnets line is by touch not looking.