Yes there is actually. I have a round magnet opposite my focuser, but on the mirror end of my OTA. I used duct tape to keep it on the lower OTA and it has been there for three years now. Simple, but effective!
The easiest way I've found to add counterweights to a Dob, is with a fanny pack / sports pack filled with sealed plastic baggies of lead shot. Strap it below the OTA, close to the bottom end. If necessary to make sure the pack won't slip down the tube, place opposing Velcro strips on the back of the pack and on the OTA. I doubt very much if there is any easier, simpler, more compact and more logical way to add a counterweight to a Dob OTA.
Very good. That's the equivalent of the primary counterweight I have toward the bottom of my OTA. That's where it should go.
The weight shouldn't go on the back of the OTA to be shifted up and down the tube every time you reposition. I stopped doing that many years ago and haven't gone back.
But try just that one weight if you have a 70mm finder, Telrad, dew shield and maybe a binoviewer or filter wheel or heavy ES eyepiece in the focuser. Then you might also need a counterweight directly opposite the finder, etc. to compensate for that cluster of stuff. Otherwise when you point the OTA toward zenith, the scope might move in the direction of the finder, etc. And then if you take your big heavy ES eyepiece or etc. out of the focuser, you'll have to compensate for the loss of that weight. That's where the magnetic weight right by the focuser comes into play.
Well, all this works and gives me smooth, fluid motion of the Dob mount without altitude detention springs or other gizmos.