I've used 4 mm MDF, PVC pipes, tennis ball, and plexiglass, not sure if that's actually the name for it internationally, lol. It is fully working with magnets inside. Got the ring and stuff on a website here. For the rest a lot of painting, not really pleased with the painting, but for the first prototype, it has to do. Now thinking of making more, and even sell some thru ebay, if i can think of a good price, wich i think is hard. And not sure if there be a market for them..
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As do i, was searching for a compass forever, but not finding a good one, so i decided to build my own. Hard enough. But worth the time!