This Rikon Kuhn cutlery tray is the perfect minidisc storage rack (without cases). The grooves at the bottom keep the discs from sliding. Sometimes you just need a bit of outside-the-box thinking. :
3D printed minidisc storage case (thingiverse link in comments) : r/minidisc
Minidisc MD Storage Box | Others | Hifi Furniture | Others and Accessories | Spring Air
STL file MiniDisc Box 10 📦・3D print design to download・Cults
Black MiniDisc tray (Holds 60 discs not in cases) storage/holder/stand/rack/ box | eBay
Genuine SONY wooden rack for MDs (MiniDisc) accessory box | eBay
Minidisc Storage Box by Gracro | Download free STL model | Printables.com