Why not connect battery negative to chassis?
Using the chassis as a common ground is a typical method used in many applications and if done correctly is generally acceptable.

Two things to watch for however.


Grounding straps should be used wherever there is a joint. Do not rely on bearings or other flexible parts for a conduction path. A flexible strap should be securely attached to the chassis on either side of the joint.
In some case an electrostatic discharge to or from the chassis can cause more delicate systems to reset, lose control, or in extreme cases, fry. Make sure the chassis is grounded as close to the battery as possible, not through some other wiring or board.