The most efficient magnetic shield shape is a spherical shield but this is difficult to manufacture in production terms. The next best is a cylinder with closed ends. The rounded surface of the cylinder is conductive to the absorption of magnetic flux. If the cylinder does not have end caps, the ratio of the length to the diameter should be 4:1.

If space is at a premium, as well as being more practical for mechanical reasons then a flat sided box fabricated from sheet metal will be the next best option. The corners need to have a large bend radius to minimise flux leakage.

The least favourable shield shape is a flat mu-metal sheet because a flat sheet only covers a portion of the flux path.
In terms of shielding size, the smaller the shield radius the better it will be as a magnetic shield.

It is necessary to ensure magnetic continuity whenever a shield is produced from a number of mumetal pieces, i.e. with lids, overlapping seams. Continuity can be achieved mechanically using friction or via welding.

If the magnetic shield needs holes then the size of the holes should be chosen with care. As a rule magnetic fields can travel into any opening a distance of two times the hole diameter. Shielding tubes can be used to protect holes with large diameters.