I have done a couple of custom 3D tanks, utilizing body filler mostly, then carving out. I don't use loads of it, but if one side of the tank has the design, can it offset the balance? I just do not want to create a design that would create a problem for the rider.
Thanks
The difference in weight one side to the other will be small. You could test this by hanging a tank prior to the modelling process ie before you start cutting filler, from a string and see or mark the lowest point. Once complete, hang again and see the difference... I would hang from the filling cap hole...
Just a point, the fuel moving around is much greater than the filler difference...
Make a filler cap adaptor with a secure fixing or hanging loop - last thing you want is an accident...
If the quantity of putty is relatively small relative to the weight of the motorcycle and the designs are symmetrical to center line of the tank there shouldn't be an issue with balance. Designs that affect the profile of the tank contour would affect the aerodynamics of the motorcycle. Then the only issue is increased fuel consumption.
My background is similar to yours with regard to design (Architectural in my case) in the Old School days. Largely retired from the field.
Side-to-side imbalance in a motorcycle will result in the motorcycle traveling down the road in a straight line, while being not quite at 90˚ to the vertical. (Because roadways themselves are usually not horizontal, being tilted a bit away from the middle, a more accurate statement would be that the bike will lean towards its lighter side, so the bike's center of mass remains over that point on the ground which is between the centers of the two tire contact patches.)
Putting more weight in one saddlebag than in the other demonstrates this easily. Unless the weight difference is more than 6kg or so, the rider's not apt to even notice it.
