Where do you find this? I don't see it on their site. Do they have it for square cubes?I've been using a Chalk Magnet's ”Red Cap Chalk Holder” For about a week and it's the best holder I've used to date. It's a magnetic reusable chalk holder. The cap on it is a nice feature and keeps things neat.
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It comes with filler disks (best I van explain them) that raise the level of the chalk in the holder to your desired height. The contact on their website can explain things much better that me.
I'm not sure of the price as I got it along with other parts and chalks. There were enough spacers to raise the chalk as much as I would need to. Coins would work just as well though.