Happy New Year!
Do you need to add tolerance in the vertical direction as well? The width is 19" so 19 1/8" or 19 /14" is recommended. The height of 8 space would be 14" so would you make the opening 14 1/8" to give a little wiggle room?
I built this slightly slanted 18u earlier in the year. With all the gear in there's no need for supports. It's just 18mm MDF. I have the sketchup file if anyone wants it.
And another view that shows part of the 12RU three-bay rack/producers-desk for other outboard.Cheers,jb
Pocket holes are great, but i prefer biscuit joints.