What is the size of your mug/cup?
I use mugs to keep my soap in. I will load the soap onto the brush, then lather on my face. You can also build a lather in a mug or certain, wide mouthed scuttles. For a long time I used a ramekin as a lathering bowl. I think hard, waterproof surfaces will be fine for a lathering bowl, like ceramics or glass. I have even seen horn bowls.
Here is an Old Spice mug. The soap puck is underneath the foil. It is narrow and shallow.
Attachment 258314
Compare the mug to the scuttle below. The scuttle is wide and has ridges on the inside of the bowl to help with lathering.
Attachment 258315