i use disposable paper cups to hold glue, filler and finishes. They’re inexpensive, readily available and easy to dispose of. Their only drawback is that they are very lightweight and prone to tipping, especially when drawing a brush over the rim of the cup to remove excess material. I’ve made several no-tip cup holders that work well in my shop.
and cut a hole in the center sized to accommodate the cup of your choice. You can cut the hole with a hole saw, Forstner bit or scroll saw, or just cut it out on the bandsaw. (If you use the bandsaw or scroll saw, you can adjust the saw table to angle the sides of the hole to match the shape of the cup.) Then hold the block upright and cut out a V-shaped groove along one edge of the top to use as a brush holder.