Happy to hear that you would like to build a mag-trim.
1) If 1/4" magnets are good, would the 1/2" ones be even better, especially for taller items; or is 1/4" more than sufficient?
The 1/4" magnets have quite strong pull when stuck to the steel. In fact three would have probably done the job. However, they are easily adjusted into place by dragging starting with a bit of a twist.
2) Have you made the extension rods for taller items yet, and will they fit within your current wood squares?
I hope to use 1/4" dowels plugged into a 45deg. block that will glue to the top of present blocks. Maybe next week if all goes well.
3) You suggest countersinking the magnets, and they do appear sunken to the surface of the wood in one photo. Countersinking would provide more protection for the magnets I would imagine; but in the last photo, there appears to be a space between the paper and the top piece of wood, suggesting the magnets may be glued onto the surface of the wood. This would seem to be easier than trying to align counter-sunk holes with the magnets resting on the tacks. Am I mis-reading the photos? (Perhaps the third photo shows the initial process before the magnetic attractiveness inspired the last photo????
I started out with countersink, but it was a job to get the right depth not having the proper tools. I know many guys out there didn't think to marry girl carpenters being interested in other skills, so to make the project easy I gave up countersinking and just epoxy the magnets to the blocks. The down side is the magnets scratch the surface of the tin plate which in time may be a rust issue. I've thought of a couple coats of water-base-poly on the tin may offset that, but have not tried it. See attached photo, and BTW, don't forget to use the wax-paper on the jig.
BTW#2, for those of you who are selling casseroles like hot cakes, I should mention that Bailey has two versions of a magnetic system for about $90 that looks very nice and professional, but very impersonal. See link:
Thanks for your kind comments.