A pair of magnetic knife holder designs I cooked up while learning TinkerCAD.
One has screw holes for mounting and the other assumes you'll be using double-sided tape. I used 3M Scotch 4011 heavy-duty outdoor foam tape to mount mine and it's not going anywhere any time soon.

The magnets are the standard NdFeB 60x10x5mm you can find on Amazon. Mine measured 59.04mm x 9.5mm x 4.67mm so those are the dimensions I used for the slots.

WARNING: These magnets are very strong and fairly fragile. They may not kill you but if the geometry is just right, they'll pinch skin and make you wish they'd killed you. If you let them SNAP together, you can break them. Be responsible and post photos of your injuries to FaceSpace and InstaTube if you aren't careful enough.

Thingiverse and TinkerCAD don't seem to be playing nicely right now so I can't link the design with my account. Here's a direct link to the design.

Creality

Ender 3

Doesn't Matter

No

0.15

40%

The long-axis dimension is 192 mm.

I printed this in PETG but I imagine any reasonable material would work. I'd love to see somebody print this in TPU just for the silliness.

Obviously, more infill = more strength but I doubt you'd need more than 30-40% unless you're really abusive with your knives.

Post-print finish work will depend on your print quality and how tolerant you (or your partner) are of things that look 3D printed.

I used Gorilla Glue Original since it turns into a polyurethane foam as it cures. This should be more flexible and shock resistant than the usual cyanoacrylate "superglue" adhesives. Not that you should be throwing this thing around (and if you do, please remove the knives first).

The magnets will interact with each other fairly strongly even at the designed distances and they will have preferred orientations relative to each other. Do a dry run of the magnet placement before breaking out the adhesive.

Place the block face down on something ferromagnetic (stove top, car hood/bonnet, ancient PC case from way back when they were made of recycled battleship steel, etc). I used a wet paper towel to dampen each mating surface (per the instructions) before placing a thick bead of adhesive onto the plastic in each slot. Place the magnets into their slots onto the beads of adhesive.

LEAVE THE BLOCK ON TOP OF THE FERROMAGNETIC MATERIAL FOR AT LEAST 2 HOURS! Possibly more. This isn't instant glue and it takes time to cure. The magnetic attraction will keep them in place and prevent the expanding adhesive from pushing them around.