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I'm not sure what you mean with "custom cut".
Ardennes has a lot of Coticules on stock, with a wide variety of sizes and shapes. Per request and if you pay, I don't think they mind cutting a piece off a hone (as long as you pay for the original hone, I reckon).
You can contact them through their website and make an inquiry.
I requested this as I prefered the size of the Super Punjab barber hone(2.5"x4") as it works great for circular honing.
Howard Schecter did not have this size in stock. Maurice agreed to cut it for me. I requested a non natural non kosher standard grade. What arrived was a Natural kosher 2.5x6. Howard sent a Pic.I had to have it. I also re-ordered the original cut request 2.5x4 Standard Grade. What arived was a 2.5x 4 Kosher Natural.
Two reasons Ardenness treated me great was increasing the grade and selecting a Natural both times. I believe this was due to a delay. They also sent them both with orders to Howard saving my big shipping dollars. They are a great bunch.
10" is very long, but not unseen. It will be rather expensive though. I'd estimate €200 minimum.
Tiger striped Coticules, are very rare, certainly if you want a large one.
So far, the striped ones I have seen were excellent for tool sharpening. But generally hard and lacking the smooth feedback that makes Coticules so pleasant for razor sharpening, where only very low pressure is applied as opposed to tools sharpening.
When you say long and narrow I imagine a 3cm x 20cm stone. : But if you're looking for something that large, number 1 or 5 might be up your ally, and they're possibly still available. They're beautiful looking stones.