I think that could become a problem if the glass of water should get overturned and as in my case, I have lots of documents nearby.
Yes; I suppose one could use one of those tip-resistant wide-bottomed travel mugs they used to make for putting on the dashboard of the car --mine had a grippy rubber bottom to keep it from sliding off the dashboard! Remember those? These were sold before cars started being equipped with beverage holders. I see them all the time at thrift stores and second-hand shops.
Nowadays mugs marketed as "travel mugs" have a narrower base, for sticking into beverage holders.
I still recommend just using your pearls rather than locking them away. It's too easy to forget to use them-- "out of sight, out of mind." However, they are more vulnerable -- a friend of mine had her gem quality Akoya wedding strand stolen during a break-in. Her husband later replaced them, but something that sentimental can never
really be replaced. She'll always know they aren't the same ones she was married in.
