I prefer .090, but .060 should serve you well. I'm 99% sure that 67 uses that thickness of sheet on all of their bases, and 33 uses even thinner sheet on theirs (.050). Much of the strength of a bent sheet metal part comes from the amount and location of bends made. If necessary, extra structural members run through the middle of the base can make the frame even more rigid.
The lightening pattern is a matter of personal preference. I have seen both triangle based and circle based patterns done effectively; however, with .060 I don't see it necessary to do much lightening.
Also, I recommend using 3/16" rivets throughout the drivetrain, as they can be swapped with a 10-32 screw if you ever deem that necessary. 1/4" rivets are a bit overkill.
@others, If i'm wrong about anything I've posted here, please let me know
