I built a box and filled it with layers of exercise puzzle mat, which I cut with an Olfa knife to create a grid of holes into which to stick the knives. Used a template to keep the holes aligned between layers. I think the holes were 0.5"x1.5", about 4.5" deep. Weirdest part was running out ofsharp Olfa blades (this stuff it hard to cut), and having to actually go back and sharpen those tiny little suckers.
I could have used pick-and-pluck foam, but that stuff tends to degrade over time, and I'd rather not have to clean depolymerized goo/dust out of the knives. Also, it was kinda expensive to get a chunk of it big enough for my purposes.
In retrospect, it would have been less of a PITA to just buy a Pelican case of similar dimensions; probably cheaper too (I used jatoba and maple - not the cheapest woods - plus it took ages to finish). This is one of those situations where you start off thinking you're saving money, but in the end you find you've paid for it (or more) somehow or other.
However, ain't no Pelican looks this purty, if I do say so myself...
I also stuck a couple large silica packets in two of the middle holes to keep things dry.