I had the same problem.
I wanted to buy an extra set of petcocks and sensor to have it permanently installed on the safari any way.
In my cas it were the "wings" of the sensor (where the screws go through) that contacted the tank first and the middle part (in between the wings) and the tank still had some space in between at the same time. I just sanded the surface of the wings (a few millimeters at the tip of the wings and gradually less material removed towards the middle) so the sensor surface follows the curve of the tank.
It needed some repeated iterations of sanding -> mounting -> leak-testing before I got it right but now it works perfectly. No gasket needed (important to me because a solution with a gasket is more difficult in the field to fix).