Actually, I use a plain bowl right now w/ no problems--heated on a desktop coffee warmer!

It works okay, but it only gives heat from the bottom and if you don't watch it, the lather can get too hot & break down, requiring re-whipping.
Now I'm getting
reallllly interested--next shaving purchase, maybe? Only thing is how big are they (inside) compared to the GP, say using a 25-25 mm knot brush?