Where's my Shrink Ray Gun? I always lose it, I shouldn't have shrunk it.
So I will have to get Guitar Hero portable by exhausting manual labor.

At this point, please don't come up with "But there is already a portable Guitar Hero - Guitar Hero On Tour". Yes, it is portable.. But does it look like a guitar? Naah. The game is good.. The handling is.. well.. circuitous.

And this is the main (basically it is the only) component. The rest consists of cardboard, white glue and wires.
No woodworking, no welding, no sewing, no complicated tech. Hurray!

Step 1: The Materials

You need:

A (working) Nintendo DS Lite or DS Classic - DSi and newer won't work with the Guitar Grip. (I will not put that on the cost list, as it can be furthermore used normally)
Guitar Hero On Tour (same)
A Guitar Grip - I got mine pretty cheap - 2$ in the Guitar Hero On Tour Decades Bundle (I bought a few more, as you can see). I will charge 15$ for that, I think that's realistic.
5 wires in different colors, isolated, conductive (surprisingly) - 2$
Thick corrugated cardboard, about 1 m²- 0,50$
White Glue - 2$
Yesterday's newspaper - 0,99$
Colors (at least black) - 2$
Shoelace (optional)

And the following tools: jigsaw, glue stick, file, brushes, cutter, screwdriver (small), soldering iron, collet

---------------------------
Overall cost: 22,49$

Step 2: Opening the Guitar Grip

The guitar grip is surprisingly modding-friendly. The screws aren't even some kind of magic tri-wings, they are boring cross recess screws, so every hobby crafter can open the device with its 13 screws (Take the transparent shell off first - obviously). You could throw everything away but

The board with the 4 buttons and its plastic cover
The bigger DS classic-adapter
The plectrum-stylus
The 2 sheets with the extremely cool stickers

Step 3: Sketching the Guitar

Put the DS-classic-adapter in the slot and copy its shape from above (DS opened) on a DIN A3 paper. Then draw a guitar shape around it that is big enough to hold the DS safely. I chose a classic (yet kind of boring) style with 2 horns. It is 32 cm long (12,5"), 13 cm broad (5") and 2,5 cm thick (1") , which is a nice and playable size.

The hole for the DS should be as broad as the DS (13,2 cm / 5,2") and as high as the DS (1,25 cm / 0,5") but a few cm longer, to get the DS out of it.

Using the plastic piece, you can add the shape of the button bar on the neck. Do also mark the line where the plastic part gets higher.

Step 4: Shaping the Guitar

Transfer the shape on the cardboard and glue 4 - 6 pieces on one another with the glue stick, depending on its hight (mine is 1/4" thick - So I need ...*calculating*... 4!)

Attention! You will have to split the guitar in half horizontally after cutting the shape, so don't use too much glue in the middle.

Cut out the shape with the jigsaw, which works surprisingly well.
Then split it in half, as mentioned, and remove the hole for the DS and the hole for the button bar from the top layer, and the middle part of the neck (with smoothened edges, you can cut those out with the jigsaw) from the bottom layer (see pictures). The button bar gap isn't very hard to cut out: Simply cut along the lines deep enough and peel the paper strips off. Remember to cut a hole for the wires.
Then draw a canal for the wires with the pen, scratching into the cardboard.
Before you glue the parts back together, lay 5 (distinguishable!) wires of 35 cm (14") along the canal or despair of the challenge to get them through the canal when it is already glued.

Step 5: Soldering

I am not a great solderer (obviously) but apparently I made it work.
Begin with the adapter. Remove the 2 nasty plastic walls and the 2 nasty plastic noses with the collet, repeat 5 times with the 5 contacts:

Isolate 1/6" of copper
Hold wire end in front of contact
Bring a bit solder tin with the tip of the hot iron on it and fix it

Not too hard.

Pull the wires through the hole in the neck and repeat the soldering with the other end.
Be very aware of the right wire on the right place!

Then try if the wiring works, plug the DS in and play Satch Boogie.

Now you can glue the electronic into the neck, but it is very important that you lay something like small pieces of wood under the circuit board to heighten it, or the buttons will hang loose. After that, hide the wires with crepe tape under the surface. Leave enough wire for the adapter that goes into the DS to move.

Step 6: Shaping

Now grab your file and rasp away the rough edges. This works with cardboard! I was surprised, too. But first, cover the adapter with tape.
You may want to file away mostly at this spots:

The 2 curves between horn and neck, both sides
the bottom part of the neck
the bottom part of the body

You should file every edge and also straighten the neck. It doesn't have to look beautiful, because it is smoothened in the next step.

Step 7: Papermachenizing

Grab a huge brush and cover your guitar with 2 - 3 layers of white glue and newspapertatters. Do this very carefully to prevent harming the electronics and use small and narrow pieces especially on the horns. Remember the path for the wires on the adapter, about one inch long (The adapter should at least be able to be moved).

After few hours, the glue gets hard and transparent.

Step 8: Finishing

I can't help you with the design, just color it how you like it! The neck is conventionally black, and I added a silver and a black line on the outside. I kept the look of the newspaper which is kind of.. intellectual. *cough*
To hide my ugly soldering job, I wrapped red tape around it. Maybe you want to do the same..?

After the color has dried, you can put the lovely stickers on the guitar and slap some more glue over them to get them under the surface.

Finally, you have the option to put 2 hooks into the guitar and attach a shoelace that helps you carrying it. I can recommend that, because it is very body-heavy, due to the DS. And then...

READY TO ROCK! _yeah!