Most of us are probably using CAD software to design our PCBs but if
you're still scribing patterns for drilling and cutouts directly
to metal during case construction then you should consider creating
CAD templates, printing and pasting them to the metal instead. Even
if you don't intend using a CNC setup, these templates make things
more accurate for component placement and allow for efficient
repeatability in case of screw ups. Also, the same template can be
used as the basis for a dimensionally accurate "decorative"
finish.
Freeware PCB CAD software abounds and while they may be intentionally
limited when it comes to actual PCB design, generally, there is no
limitation when used for something like case design if you choose the
right one. In other words, provided devices such as pads and "known"
components are not being used, your design can be any size.
The link below is a set of pictures demonstrating the use of CAD
software and printed templates to CNC drill and center drill
(equivalent to center punching but under CNC) a 12" X 6"
sheet of aluminum that becomes the front, back and base of the case.
Holes over 1/8" were drilled manually after center drilling.
Cutouts were nibbled using templates as bounds. Finally, a decorative
finish printed on self adhesive film was then used for the front and
back.
http://www.pbase.com/eldata/caseconc