Buy 3 allen head bolts 50mm long @ M5 x0.8 pitch spacing between drill hole in flat plate 21mm
Buy a bolt 65mm long with at least 45mm of thread plus a nut.
Tack weld the nut or thread the plate to take your bolt centre of 3 three drilled holes.
20 minutes is all it takes to make one :-)
hint ... you can use your mag as a template for measurements.