When I collected Tau I had a similar problem - I had one tank that had three stem tips rattling around in it. I eventually gave up and sat the tanks on clear plastic cases I found at my local hobbystore - they're the same material and size as those Chessex dice cubes, but with a clear lid. The wider area meant they were more stable, and if the tank was trying to hover somewhere where terrain would tilt it, or the longer base wouldn't fit, you could flip it 90 degrees and make it fly a little higher with a smaller base. Worked great, but I've since sold off my Tau.