I made a brush like that once to clean out a tank. I used a cone wire brush for a die grinder then weleded it to a longer steel rod at about 16" in length .... this let me run it on a pistol drill and fish it down into the tank.
Certainly not pretty and not stable to run at crazy high RPM .,.... but it worked.
I think I've got a plan that involves and apparatus that will attach the tank to the rear wheel of my garden tractor then with some dry wall screws in the tank run the tractor on jacks at low speed in first gear