Walsh songs aren't stomp-box type effects so much as a speaker adapted to bounce the output from your mouth into a microphone.
Step 1: Salvage
This project is best suited to small speakers (1-4"). I used a Kenwood computer speaker & started by removing the ported portion with a hacksaw.
The speaker is 3.5 inches in diameter, 6 ohms & rated to handle 20
watts. It's an adequate match for my DIY Champ's 4 ohm / 6 watt output.
Step 2: Enclosure
I wired the speaker with a proper 1/4" cable and closed the void with a piece of scrap aluminum. Plastic or wood would have worked as well.
I added a long funnel ($4 at auto-parts shop) to the speaker using flexible glue & plenty of duct tape. Then I glued a 5' section of clear 1/2" rubber tubing to the end of the funnel.
Step 4: Isolation
Sound isolation is important, since you want to get as much sound as possible at the tube-end of this rig and as little as possible from the speaker enclosure. I used some medium-density foam rubber and, yes, more duct tape.
Step 5: Finish
I applied several layers of green towels (green being the most sound-deadening color) & finished by placing the whole thing in a black pillow-case. It's bulkier than a pedal unit would be but at about $20 and an hour or so of my time, I'll take it. It's loud enough for stage use but not "knock your fillings out" loud. There is no high-pass filter like most commercial units but with my amp's tone knob turned up, it is fairly balanced.
Once hooked up to an amp & guitar, the tube end is velcro'ed side-by-side with your microphone. The tube end goes into the corner of your mouth, allowing you to shape the sound, which bounces it into the microphone.
"I wonder how you're feeling, there's ringing in my ears..."