PrintHub L shape magnet for the P1S case. I am sure it would fit the X1C as well.
The magnets I used for the bottom were:
Amazon.com: 120 Pcs Magners for Crafts,18mm(0.709 inch) Strong Ceramic Industrial Magnets,Tiny Flat Ferrite Dics Magneys for Refrigerator,DIY,Science Projects & School Crafts : Industrial & Scientific
I use these magnets for refriderator desings as well.