I’ve been using a magnetic bed on my Ender 3 that I pieced together for a week now.
Removed the orginal Ender 3 bed surface.
First layer is a Magnetic sheet from McMaster Carr pieced to cover entire bed .
Adhesive-Back High Temperature Flexible Magnett (to 120C) sheet 9x12
catalog number #7301T21 $14.85 +shipping
(learned about this by reading a post by Joe Arndt on Ender 3 Facebook group )
Second layer is Spring steel sheet
Additional Buildtak FlexPlate 241x251mm size for Prusa mk3 $25.75 + shipping
Oriented so that long dimension is front to back (makes a nice handle to lift and remove. FlexPlate) cut out positioned to right side. Made a spacer in Tinkercad to move power supply to the right to get maximum width between Ender 3 uprights so that placement of FlexPlate side to side is not so critical
Third layer build surface
TH3D EZMAT print surface 235x235 $10.99 + shipping
Applied so that back edge is flush with back edge of FlexPlate and centered side to side.