Yeah, If he would seriously try to
put in in that controller, then...well... It probably wouldn't look too good. I believe the best thing he could do would be something like
this.
The two halves of the controller on the corresponding sides. Except, with a psone controller, and a gba sp in the middle.
It only took me a good 4 hours I think. But I have experience with modding.
Let me see if I can write a general tutorial....done. Begin reading.
Basically all you have to do is hook the contacts from the buttons to the controller. Now, regarding connecting wires to the sp...Look here.
Notice anything? Anything you may have not considered? Now, look closer.
Still dont see it? Lemme help you out!
These are your buttons. They need to be extended to the controller. But...wait....HOW?!?! There are no contacts you can solder to!

This is madness! Well, look here.
These are essentially just a different design of tact switch. You will have to remove the top film on these. Underneath there should be at least two contacts. You will need to solder a wire to each and extend that wire to the board in your psone controller. Hopefully you will have cut up the controller board in the controller and removed any resistors or such, and only left traces and contacts. Components like chips or capacitors can make the controller send incorrect signals, or possibly damage the sp. I will see if I can find a good pic of an empty controller board to show you what I mean. If you own a multimeter, you should find out if the buttons share a common contact, so you dont have to solder as many wires.
Did I forget something?

Hmm, well....maybe. Its something here...

Aha!

The L and R buttons!

These are two regular tact switches! That makes things easy! (The picture does not show this, But I have opened an sp before!)

You can see the four contacts! But wait, Its not really four, is it? Well, no. Two are used to help keep the tact on the board, to keep it sturdy. Use your multimeter, and find the two that are connected, maybe a third if the common is ground and the reinforcement is also connected to ground.
From here it should be simple. Just connect the wires to the unpopulated(no components) psone controller. If all the buttons share the same common, on both the sp and the psone controller, then just connect those, as it would make things easier. Unpopulating a board is simple. Use a soldering iron or desoldering iron to remove the components. Or just break/scrape them off. If the components completed a circuit, a small blob of solder or a small strand of wire should be used to complete it, if it makes it easier.
Then, onto case work.
I have been told, and it is an opinion shared by many that I am not good at making beautiful cases.

However, I can make a structurally sound one.
If you decide to do the two halves one, remember this: You want to feel like you are holding the unmodded controller, be it a little more wide. You will not want to touch any epoxy, hot glue, or open plastic with your fingers during use.
Ok, you will want to have the two halves connected together in some way other than the sp. I recommend that you get a small flat piece of plastic, the length of the extra space between the two halves, and epoxy it to both. Preferably in the back, or maybe the front, or even better, both! You will want it to feel like a solid unit. Gluing the halves to the sides of the case and having no support is not a good thing.
Look here:

This is the back of my unit. There are two things I will point out

I said look here! No, Closer!
Ok, the red circles are two pieces of plastic, glued on to eliminate unnecessary holes or openings. This keeps the guts inside safe from harm, and alien material out, and if you spend time and use epoxy, It can become helpful and cosmetic!
Now, the green square is my flat piece of plastic. It holds the two controller halves together, adding more support, which is necessary. It, along with the two hole covers, provides a more sturdy controller to hold, and, done correctly, can look very professional.
In my unit, there is no flat piece of plastic in the front, as the screen would be blocked. I would assume that if you did it with the sp, you could not place one in the front either, as it would keep it from closing completely and correctly.
I hope that this information will help you, and if you have any questions regarding this material, or other techniques, just post again and I will try to help you as much as I can.
SS
