Finding iPhone cases with magnets that don't affect compass

Navigation Problem due to Magnets in Case/Wallet.

Cases w/ magnets: There are lots of iPhone Cases / Wallets out there, which do use bigger (or an array of smaller magnets) to fit closely.

Problem w/ Navigation: My personal experience is, that some do interfere with the internal compass badly, so low speed (walking etc.) navigation is often "upside" down.

Question: How can i shop for the ones, which do not produce this kind of problem? Are there certified / endorsed ones. Is there a list?