Quick answer: For strongest DIY trays, put neodymium magnets in bases and a ferrous/steel sheet in the box. Magnetic sheet under magnetised bases is usually the weaker pairing.
Recommended pairing
Magnet in base + steel in box = polarity does not matter and hold is strong.
Build guide: DIY magnetic case.
When magnetic sheet wins
If you glue steel bottoms onto many bases and want one flexible liner, magnetic sheet can be cheaper — accept a lighter hold.
Frequently asked questions
Can I stack two magnetic sheets?
Avoid magnet-to-magnet sheet sandwiches; poles misalign and models jump.
Rubber-steel sheet?
Often grippier and quieter than bare tin; still counts as ferrous.
In practice: magnets in the base plus a ferrous (steel) sheet in the tray is the combination most hobbyists use for club nights. Craft “magnetic sheet” is often weaker and suits light models or temporary trays. Match sheet size to your box base, and test one magnetised model with a gentle shake before you trust a full army.
Sizes and grades: best magnets for miniature bases · is magnetising worth it.