Anyone with a McWire 1.2/variation - help?

Hi, I have been sussing out the design of the McWire and have a question -

does the back board thickness ("vertical base") have to be thicker than the x/y/z stages because you need to countersink the large heads of the bolts holding the flange?

Can anyone with a McWire bot check this for me?

Cheers,
Hazel