The Slide and Body
The "switch" that makes this trombone work was inspired by Andrew Carle's design that uses a wire with its insulation removed sandwiched by two pieces of copper tape with conductive adhesive.
The trombone is composed of two parts: a slide and a body. The slide attaches to the body by sliding onto a smaller cardboard tube that is part of the body.
The slide acts as the ground, or Earth, while the copper tape and wire pads on the body connect to the MaKey MaKey's "keys."
The Copper Tape Pads
A series of copper tape pads were built on the smaller cardboard tube on the trombone's body.
After one set of pads was built, holes were punched in each pad so wires could be routed through the pad. The wires were soldered to the copper tape pads.
Afterwards, the soldered wires were sandwiched between the first pad and a second copper tape pad placed atop the first.
The trombone is connected to a computer with the MaKey MaKey. The trombone is programmed in Scratch to play notes.
A disclaimer: I do not play the trombone in real life, so instead of the actual slide positions required to play the note that corresponds to its place in the scale, I just placed the notes in succession along the trombone's body. When the slide is all the way in and the ground is bridged, the trombone plays its highest note.
You can remix the trombone project on the Scratch website.