...easiest way is to paralell the gamepad buttons with a custom harnes. First I would look again and decide what peripheral is best to use, any USB input device will work could it not? ie. a USB num-pad could be used - IMO use the cheapest peripheal you can find as these usually are built in a crude manner and are easier to work with.
Whichever device you choose to donate to the project is up to you and the principals of how to make it work are the same. Available from any electronic stores are micro-switch's, two of these can be mounted to the padels, the G25 has it's padels mounted on a small hinge at the shaft of the wheel, when the padel is pressed the micro switch closes, that's simple and the only factor determining wheather or not you can replicate this setup relys on you ability to machine/make a sturdy padel and switch assembly.
Wiring is simple, all you'll need to do is connect each of the switches to Some new wire and back to the pcb of the device at whatever switch you want to, this way the device can be tucked away somewhere else.
My final suggestion is best, it will take much less time to assemble as the use of padels is not nessessary.
Touch sesitive sequential shifter! Instead of micro switches all you'll need is two small conductive metal plates to press with your thumb or finger. again available from any electronics stores are transistors, all you'll need to do is wire back to the pcb of the device as before but this time when you touch off the collector of the transistor the circuit will close.
...or just don't
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