A simple way to make a cup for a wet paint brush, is to cut a plastic bottle in half. The smart thing here is to keep the top half, with its cap on, and place it over the bottom half. This way it holds the paint brush in place, and keeps the smell of the solvent from spreading.
Step 1: Cut the Bottle in Half
Pick a plastic bottle of suitable size for the paint brush. Cut it roughly in half with a knife or scissors (keep the cap on!). Try to cut it where there is a bulge, so that the top part can be made to fit over the bottom part.
Step 2: Put It to Use
Fill the bottom half to the height of the brush part, with whatever liquid you use. Then put the brush in, and put the top half on!
Step 3: Ready! Be Careful!
Your paint brush preserver is ready. Virtually no smell leaks out (and thus no evaporation losses). But be careful with the contents, don't lift the bottle with the cap, and don't topple it over!