There was a discussion about mass here at one time....area has more to do with it than mass, but mass is also important too...if it was me I'd stick with the 1/8"....any copper rectangle out there??  
   Also, how are you going to layout the holes? You can't layout or transfer from the amp side, you'll wind up with a mirror image and the "tilt" of the outputs will be opposite of what they should be....I've tried it. I had to lay out mine from the transistor side by clamping a stock heatsink to the copper plate and using a transfer punch. A real machinist could could drill the holes on a mill with a DRO but man, that's a lot of work...