Step 1: Supplies

Resin-casting compound. We like Smooth-On Smooth-Cast urethane resins for low odor, quick cure time, and ease of measurement. Smooth-Cast 65D or Smooth-Cast 300 provide a nice surface that is durable and non-brittle so that it can withstand a bit of wear and tear. However, any resin casting compound is fine. We created a rainbow assortment (great for classrooms and workshops) using So-Strong pigments.
Mold-release spray
Magnet-friendly pan for casting. It is crucial that magnets stick to it. The pan in this example is a 7.5" diameter shallow baking pan that we found at a thrift store. Small toaster-sized pizza pans work very well, and it is easy to pop the boards out at the end.
Measuring/mixing cups and sticks
Small, strong magnets. The ones in this example are tiny, strong 4 mm diameter x 3 mm height magnets. The dimensions can vary, but the magnets must be strong enough to attract and hold stainless steel conductive thread. We make many ThreadBoards, so we purchase magnets in bulk from AliExpress, but there is a long wait time for shipping. K&J Magnetics is a good US-based supplier for smaller quantities.
(optional) Template for magnet placement. We cut the templates above using a laser cutter, but it is fine to use a hole punch, Cricut/Silhouette cutter, or no template pattern at all.

Note that your ThreadBoard can use any shape/size and magnet configuration, though it is useful to have the magnets close enough to easily attach the two legs of an LED.

Step 2: Magnet Placement

Spray the pan with non-stick mold release.

If using a template, place it into the pan, and then position the magnets. Be careful to keep mold release off the parts of the magnet not attached to the pan (or the resin won't adhere to the magnets)!

Either position the magnets freehand, or create a template like the one attached. You can design templates using a cutting machine like a Silhouette or Cricut, a laser cutter, or a hole punch.

Carefully remove the template from the pan.

ThreadBoardV4.pdf
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Step 3: Resin

Prepare the resin according to manufacturer's instructions. For a 7.5"-diameter ThreadBoard, we used approximately 90 ml of (each) Smooth-On parts A and B.

(We found the volume of a disk 3.75" in radius, .25" thick: 3.75^2 * pi * .25 = 11.04 cu. inches, which would be around 180.9 ml). Or, you can mix up a batch of resin and pour to desired thickness. :)