To have my whiteboard pens just on the board I designed a small dual pen holder. This pen holder is attached with four square magnets to the board. If you find 7mm x 7mm magnets you can press them tight-fit into the mounting wholes without any glue they. If you happen to find smaller magnets you have to glue them into the wholes. I use expoxy glue for this.
I printed the file with Cura, using 20% infill and 80mm/s speed on my UM2
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