In addition to using diecast aluminium boxes i have also made a few pedals in ABS boxes. They are much easier to drill and work around with. Only problem is they feel very light. Now, i can think of the following solutions if anybody has any more suggestions please let me know.
1- a number of commercial pedals use the upper casing as plastic while the back lid is metal making it heavier. I cant get hold of just metal back lids, would need to sacrifice a whole die cast box which isnt feasible
2- putting in something heavier like a chunk of metal inside the box..

but that will look quite phony when the box is opened..
anymore suggestions guys?