The Caladius Grav-Tank shares its sprue with the Pallas Grav-Attack, and most of the magnetization work centers on the turret swap rather than the chassis. The notes below cover the relevant magnets and a couple of basing options carried over from the kit, then point to sibling guides for the rest of the Adeptus Custodes range.
Use the Medium Flat – Short Magnetic Flight Stand.
[Buy Product=2x-medium-flat-magnetic-flight-stands,variant=14030250704980]
Use 1 Large Thick 6mm x 2.6mm Flush-Fit Base Magnet to magnetize your base to a magnetic transport case.
[Buy Product=gw-flush-fit-base-magnets-large,variant=39439165882452]
For a replacement base with preset magnet holes, use this one so the magnet never comes out.
[Buy Product=bases-premagnetized-warhammer,variant=30427499921492]
For a peel-and-stick basing option, use 3 of the 25mm Peel & Stick Inner Base Magnets spaced in a triangle inside the base — no drilling required.
[Buy Product=flexible-self-adhesive-magnets,variant=51483453784354]
To magnetize the turret, use 25mm x 2mm disc magnets. They are strong, sit flush in the existing turret hole, and need no drilling or centering since the magnet diameter matches the recess. The same magnet works for either the Caladius spirit-shrine turret or the Pallas open turret if you build the kit for a dual configuration.
[Buy Product=4pcs-1-x-1-16-25mm-x-2mm-disc-magnets,Variant=All]