Maybe your existing choir could spread themselves out at different Masses and deliberately sit in different places, and find a few new voices that way. People often respond to being told they sing well. Try a little humor. I then used anonymous excerpts from these to create a flyer,distributed as a bulletin insert. No matter that I still felt we needed 6 more.
People like to belong to a successful endeavor.
Reporting how well it's going instead of moaning about how we need more people might get people thinking that they are missing out on something by not joining choir.