I use a small dish about 15 cm round to grind the borax.
There had been a number of references to a small dish and slate
dishes for grinding borax. From the descriptions given the small
china-ceramic dish, or in melamine ware, used for serving soy sauces
and other chinese condiments would seem to be a suitable utensil for
this purpose. They cost just a few tens of cents in any chinatown