I took the concept from araymbox and created my version of the GamePi for N64 games. This one includes a psp joystick and an extra two buttons on the right side. I also reoriented the ports to the sides of the case.
The battery I used was one similar to what araymbox used, but it isn't in stock anymore so not sure how helpful these files will be. This was built using araymbox's instructables guide which can be found on his GamePi 2 project page.
Some of the things I swapped out:
2 of Mini Metal Speaker w/ Wires - 8 ohm 0.5W
IDS 2 Pcs PSP 1000 Joystick Analog Replacement, Black
Gogyre Power Bank 10600mAh External Battery Pack