[asid61]

Steel is not worth it usually in FRC robots. I would only use it for weight, like on flywheels.
1/8" 6061 aluminum is super strong for a WCD. Lots of teams even use 1/16" 6061 for their robots.
Keep in mind steel takes much longer to machine than aluminum, so it's easier on the machinist if you just stick with aluminum over steel. If you need the weight, add some plates, but I don't think you'll need it.

Why do you need weight?