This is a Talkbox. Not necessarily for a guitar, but that's certainly where I've seen them most.

Most designs seem to feature a horn tweeter with a long piece of garden hose reaching fro the floor. I decided to make mine pocket sized, so it has a small speaker, and a short piece of hose.

Essentially, it's a means to attach a piece of pipe to a speaker. It's a box containing a 4W speaker, connected to a piece of garden hose.

The theory is that the sound travels up the pipe, into the player's mouth, where they can shape it, like Ritchie Sambora https://www.youtube.com/watch?v=tXN22O_R9GQ.

The speaker I used was a Maplin part N25KJ 28x40mm oval speaker. The base could easily be remodelled for other sizes / shapes, though.

Use of this device will require an amplifier. I suggest you connect the device to the headphone output of a regular amp or maybe a pocket-sized practice amp. You may need a microphone and a second amp, then to amplify the sound, as it effectively comes out of your mouth.