Nice^ ! Yea you can make one super cheap out of pvc and line it with the 1" thick egg carton shaped foam and glue in in place. Pretty much copy the design of a duct muffler out on the market.
I'm using duct mufflers on all of my fans. Pulling air through the filter and blowing out through the muffler. I think I'm also going to build some small boxes around the fans themselves out of the thick foil lined styrofoam board from lowes.