The steel disc is glued to any 12" bat that already fits your wheel. I stuck mine together with strong double sided sticky tape. Four 1/4" Neodymium magnets are countersunk and epoxied to three 1.75" square pine blocks. (See photos) Dimensions are not critical, but it is a bit of a trick to keep the magnets from jumping together during assembly. The blocks were faced with scrap foam Gripper Pad, but any medium stiff foam can be used.
So far the magnet trimming holder works well and is somewhat easier than using clay wads. I have yet to make up a set of extension dowel rods for holding tall pieces, but will get to it soon. This device is about 1/10 the cost of Giffin Grip and can be used on pots of any shape. Good luck and let us know how it goes, and if you have improvements. Please see edit below.
11" Steel Quiche Pan:
Hardest part of project is epoxying magnets, so I've added a magnet assembly jig. The photos show how to use it to make the three magnetic blocks. When chuck is finished first center your pot then slide the blocks to hold the pot. Not much pressure is needed.
To all members who are thinking of building a Magnetic Trimmer,
My attempts to build simple extensions for trimming tall pots were complicated.
Another approach I use successfully is to throw a double-ended trimming chuck and bisque
it then center it on the mag-trim. (See photo) Dimensions are not critical.
I use a bulls-eye level to level the pot in the chuck. (This assumes your wheel is level.)
You could also buy a set of extensions from Bailey for about $16 and screw then to the mag-trim blocks.
Good luck with your Mag-trim, and let us know how it goes.