If you buy the sligjtly more expensive ones they come with semi cut foam. Its "cubes" with a small attachment to each other. You just pull out the cubs you dont want, leaving you with perfect cutouts.
@brajner That's some serious set up you have. I love the layout. Looks like it was meant for that too.
Cool setup. I was thinking about doing something similar for my tech gear. Thanks for sharing it gives me a better idea how this work for my needs.