@AgX the narrow ring (maybe rubber band or similar?) idea actual sounds pretty cool! when I go through my next round of testing with the 1500 tank I'll give that a try and see how it ends up. My only concern is if the "bump" will tweak or hurt the motor.
No rubber band.
There are several ways to achieve this:
-) one or two partial layers of foam rubber, covered by one complete layer. The foam rubber though must be very dense to not induce too much rolling resistance
-) a band cut from plastic sheet as ABS, cemented around the tank by distance pieces of varying height
-) a circular sheet from ABS with an inner hole the diameter of the tank, but cut out excentrically, pushed over the tank
There will be no "bumb" as the rocking movement is continuous.