First print out your message on lots of piece of paper. Then, cover your entire sheet of metal in tape. Tape the paper down to the metal and then carefully cut out each letter. You'll be cutting through two layers, so press hard. Then, peel off the tape and paper and you've got yourself a custom stencil. Simply paint, and then carefully remove the tape and you are ready to hang. Use some epoxy in addition to nuts and bolts when hanging your sign so that it stays up longer.
Choose your what you say and where you say it wisely as not everyone may think your sign is as useful as you do.