i used to sell those aluminium ones and to be honest had nothing but issues with them as people would over tighten them, either damaging the threads or i saw a few cases the thread and internals stuck in the sump and the head snapping off flush with the surface.
I actually get ones custom made for me now out of steel that are nickel coated like OEM to prevent rusting but have the high strength magnets in the middle, easily able to support a 3/4" racket from just that tiny magnet.
I fit these to all the forged engines I've been doing now as a standard part. But i do sell them separately if anyone is interested. There £12.00 delivered to the door.
Tim
TB Developments