Based on other designs, I knew the basic core components of a Handheld Raspberry Pi Retro Gaming System:

Raspberry Pi

Screen

Power Management

User Interface (Buttons and Switches)

Audio Amplifier

There are a lot of plug and play options available for these functions but I wanted to try to keep things simple and reduce costs. I researched for a few days and I found that I could make this whole project for right around $100. Even less ($80) if you have some random parts on hand like tactile switches, a rocker switches, a slide switch, and/or a USB port.





Raspberry Pi

I used a Raspberry Pi 2 Model B with a 32GB Micro SD card. No reason behind it other than Amazon.com had them in stock with free 2 day shipping. I used this list to make sure the micro SD card I was buying would work with the Raspberry Pi.

Screen

For the screen, I didn't feel I needed a high definition touchscreen. This is going to be a retro game console and when I played these games as a kid, they definitely weren't HD. We can also save up to $35 on the build cost by going with a cheap backup camera LCD screen. Ben Heck used one of these screens in his Retro MAME handheld and it worked perfectly for this project as well. Amazon.com has various models ranging from $15 and up. The screens typically run on 12V (the same as a car's electrical system) but with a little multimeter probing, you'll most likely find that most can operate off of 5V.

Power Management

A few designs I stumbled upon had the battery, charging circuit, and step-up converter as separate PCB units or modules. I found Instructables user sonicase's idea of cell phone battery rechargers to be a perfect fit. For $15 you get a decent size LiPo battery cell, a charging circuit, a step-up converter, a 5V regulator, and a battery life meter all in one tiny package. Re-purposing one of the chargers saves quite a bit on build costs and the space savings help when it comes time to 3D design. With a quick hook up of two wires, you can power up your Pi for testing without the need of an external power supply as well.

User Interface

Originally I had planned on using a cheap USB gamepad I had on hand, hacking up the PCB, and soldering wires to it. During the original prototyping stage, it worked perfectly. Once I tried to minimize the PCB (Cut it up into a smaller piece) the Raspberry Pi would no longer recognize it as a USB device. After scrapping that idea, I looked into using the Pi's GPIO as an alternative controls interface. Even though I spent a day messing around with the USB controller, I was very happy to have found the GPIO option. It eliminates an extra PCB, saving space, and it works great after a short installation of Digital Lumberjack's Raspberry Pi Joystick Driver. I cover this in more detail in the wiring step.

For A, B, X, Y, Start, Select, and the directions buttons, I used some prototyping board, cut to size with a dremel, and 10 tactile switches. For the shoulder buttons I used two 90 degree through hole tactile switches. For the mute button and power button, I re-purposed some random switches I salvaged from some old electronics. I've put some equivalents in the BOM below. I also used a 26-Pin right angle connector to make the wiring to the GPIO a bit easier. This piece is optional but it helped out quite a bit. Especially if you plan to re-purpose the Raspberry Pi at a later date.

For the USB out, I desoldered a through hole USB port from an old USB device. I've also included a link in the BOM to alternatively buy one if you don't have any old hardware laying around.

Audio Amplifier

The audio out of the Raspberry Pi is at pre-amplified headphone levels. To power a small speaker, I found a small LM386 module on eBay for less than $3 and it works off of 5V. For the speaker, I used a loud speaker I salvaged from a broken Nook. Most, if not all, cell phones, tablets, and eBooks have these loud speakers in them as they are incredibly small and still sound good at audible volumes. For testing purposes I used a battery powered speaker similar to this one. This won't be completely necessary as it was used for concept testing.

Here's a list of the links to the parts in the BOM:

Raspberry Pi

Micro SD Card

Battery

LCD Screen

Audio Amp

A, B, X, Y, Start, Select, Directional Switches

Shoulder Button Switches

Power Switch

Mute Switch

USB Port

GPIO Wiring Connector

Prototyping PCB

Speaker

Case Screws