Ton-chi-ki.blogspot.com posted an overview of the PC Engine Mercurius, a PC Engine turned handheld. Functionally equivalent to a TurboExpress (AKA PC Engine GT), it features a 3D printed case, built in controller, and 4:3 RGB LCD (LCDDRV). With the exception of being extremely wide and probably very uncomfortable to use in practice, it is a polishing looking mod.
Sadly, it requires sacrificing a PC Engine and an official controller, something that I’m not comfortable with in reflection, despite my initial enthusiasm for the device. I’d much rather see enthusiasts purchasing broken TurboExpresses and restoring them, thus saving precious relics of gaming history instead of destroying them.
Tonchikiroku has been posting different builds to their Twitter feed as well.
The project is closed source and 3D design files have not been shared, but cases and parts are available for purchase at Kadenken.com
Thanks to @KonixSlipstream for bringing this to my attention.