I have successfully used utility knives with very sharp blades, a steel ruler and a vertical sawing action to cut out rectangular areas for e.g. camera bodies.
I have also used a drill press with 1 1/4 and 2 inch bits for eyepieces, to drill round holes, with the foam sandwiched between 2 sheets of thin ply.
It is not easy and can quite easily go wrong! Even taking great care, my efforts do not look as nice as the OEM die cut foam inserts, but they are functional and do restrain and protect the equipment as intended.
Here are some articles on CN:
http://www.cloudynights.com/item.php?item_id=419
http://www.cloudynights.com/item.php?item_id=389
http://www.cloudynights.com/item.php?item_id=398
The last one may be the one Striker refers too?
Cheers
Dennis