I use a cardboard box- the type that reams of paper come in.
Cut a hole in one end, including the lid. Line the bottom of the inside with a plastic garbage bag. Take another garbage bag and pack wads of newspaper in it. Place the bag inside the box. Firing over a chrono into the open end has the paintballs passing through the bag and any paint that breaks is absorbed by the paper. You can shoot quite a lot of paint into this arrangement before adding more paper or replacing the bag.
Thrown the whole mess away when the paint escapes the box.
Then make a new one.