Hi Bertie,
If you have a bathroom tile, piece of glass, or a smooth plate of metal then you are half way to sculpting your own splinter mats.
I am not sure if there was a standard size that was altered to suit by members of the crew, or each class of vessel had them made to order by a company. Given that most sailors could sew until well into the seventies, I would say that both are possible.
I mark out oblongs onto a plate of steel I have which are half inch by quarter inch which seem about right for 1:96th scale. Page one of my HMS Ready build shows this late and also some fitted onto the bridge:
http://www.modelboatmayhem.co.uk/forum/index.php/topic,50673.0.htmlAnd here you can see more fitted to the bridge of M19:
http://www.modelboatmayhem.co.uk/forum/index.php/topic,57152.0.htmlI used an arrow head shaped sculpting tool and you can make one by carving a flat piece of hardwood to shape and allows you make it the shape you want. Good luck