It's your call Jonny if you don't make it invertible - after all, Inertia XL uses its disc to help it self-right when it's stuck on its back. But having designed the first Drumroll as non-invertible, I then had to fashion some god-awful roll hoops which didn't really work and meant I went in to half my fights worried about simply getting flipped.

If I'd just spent, say, 10 minutes tweaking the design before any materials were cut, so that the robot could run both ways up, I would've saved myself a lot of hassle later on in both the build and the arena. With Drumroll II being invertible, I only need to worry about getting flipped if there's a chance it will result in me exiting the arena. Up to you though. I don't think a magnet will help that much either; low-pressure flippers (and possibly even electric lifters) will still turn you over.