I used 20x5 mm neodymium magnets, N52 with a pullforce of something like 23 kg each. There's about 1-2 mm of wood between the magnet and the knife, single magnet per knife, knives are held firmly, some do rotate a degree or two depending on how they are placed. If I'd do it again I might try a brick pattern but then using smaller magnets.
I used triple rows of 10x5 mm n52 with 1-2mm wood cover.
Hols a 240 yanagi, 180 Usuba and 160 deba but anything bigger would need to be tip down so the bolster stops it sliding off. I wanted to do double rows of 20x5 mm magnets for my bigger knives but were to expensive at the time.
If you speak to a supplier they may send you some different sizes to try, I managed to get pairs of about 6 different sizes and made some mock ups to find the right strength size and spacing.
Good luck with it