I've been wanting to do this myself, thanks for saving me the trouble! Have you run any games with it? Do you just skip over the diamonds? I thought it may be interesting to have some sort of special ruling for them but hadn't come up with any ideas.
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Yep, I've used it in games before, (or, rather I've used something very similar). It worked really well! The diamonds I usually just black out in something like Photoshop and ignore, skipping over them in play.
Something I thought of, but have not tried yet is random encounters. Pick your starting town and in the diamonds around it, put the number "1". As they radiate out from your starting town, increase the number. When players move within each ring, roll a d20. If the roll is equal to or under the number of the current "ring", you get a random encounter. Something I've been keeping in my back pocket to try in a West Marches or Open Table game. Just a thought!
Now that is a neat idea! I could see that working out pretty well! I'd love to hear your experience if you ever get a chance to try it out.