Just run a few tests with the magnets before you begin. My completed one has a medium strength grip. It will hold the knife on the left just fine, but the one on the right is heavier with a narrower blade and isn’t suitable.
Grip can be adjusted by engraving deeper, bringing the magnets closer to the front, or by using different stronger magnets.
Thanks for the tips. I’m certainly interested in making one myself.
Great tips–I have a decent selection of magnets, and should have a machine soon that can engrave marks indicating which knife sizes go where. 
Love the simplicity. Maybe a darker wood would have matched better, but this way you can be reminded of your impressive work every time it catches your eye!
If the cutout has room for 2 magnets, that would boost the hold strength, too.
knife racks are on my list of things to do. i actually cannibalized a big pile of old hard drives last year and have a hefty stack of magnets from them that have been earmarked for knife racks.