I made a magnetic base myself with a strong magnet (can hold 37 kg if you mount the magnet directly the metal surface).

https://www.first4magnets.com/pot-clamping-magnets-c43/32mm-dia-neodymium-pot-magnet-stainless-steel-rubber-coated-34-7kg-pull-p8198

magnet dimensions:
diameter 320mm
height 8mm

gcode prepared with simplify 3D
Printing need stop layer 50 and you can put the magnet inside the thing.
you can stop manually or script

{REPLACE "; layer" "M117 Layer"}; show layer number and Z height on LCD
{REPLACE " Z = " " Z="}; make room on LCD for text
{REPLACE "\nM117 Layer 50, Z=" "\nG1 X10 Y5\nM300 S1000 P1000\nM0\nM117 Layer 50, Z="};