I ended up trying this mod: using the Rockchip Wii controller "contact" pad and a DS Lite directional pad. The DS Lite directional pad is much better, but the Rockchip's contact pad doesn't fit very well. I have a newer GPD XD and its contact pad seemed very fitted to the space moreso than the older models I think.
In the end, I didn't end up using it, but I kept the DS Lite's directional pad, but ditched the Rockchip's contact pad. It just didn't fit or work well.
If someone from GPD is reading this: check out a DSi XL or 3DS XL. Both of those handhelds have similar, smaller d-pads, but the contact pad is much "clickier" and just feels better. The pad included with the GPD works and you sorta get used to it, but Nintendo's handhelds are MUCH better – the same goes for the buttons on the right.
It would be great if GPD could improve this for future models and maybe sell the replacement membranes (and directional pad!) so that people who already own an XD could upgrade their unit properly.