I designed these in onshape cad with parametric variables in mind these are 30mm wide by 45mm long for 6x3mm magnets but I also designed the untested 90mm length. I decided I like the look of 45mm segments better and the weight of cable could use the extra magnets. Originally designed for the outer track but also works on the inner track provided your rack shelf bolts or rails don't protrude too much

I printed these in Tangled PLA with 15% infill, 2 walls, and 0.2mm layer height but settings aren't that important they shouldn't need to be super strong. Magnets simply press fit in assuming your printer is dialed in and magnets are 6.0-6.05mm nominal diameter.

Link to onshape source project if you want to try modifying dimensions to support other rack dimensions.