This is my second attempt to make usable (and useful) Raspberry Pi UMPC. An upgrade in comparison to 4" version, because now connection to the screen is via HDMI (image) and GPIO (touch), so now we have nice 60 FPS with 800x480 resolution.

Required components:

• Raspberry Pi 3B(+)
• Waveshare 5" HDMI+GPIO LCD screen (double check not to get HDMI+USB, because this one won't fit!)
• 4 x M3x30 screws
• 4 x M3 nuts
• 4 x M2.5x5 screws to mount Raspberry (holes in bottom part are a little smaller, so the screw should create thread as you screw it in)
• **Raspberry Pi power supply or strong enough powerbank (my provides 5V, 2A and it seems to be enough, Raspberry doesn't complain)

Build is quite straightforward:

• Print all parts (optionally supports as well, cover is not required, but protects screen during transport, it's snap-in)
• Screw Raspberry to the bottom part
• Put the middle part on
• Plug the screen to the GPIO through the dedicated hole in middle part
• Plug the HDMI connector
• Put the top part on
• Screw everything together
• Done :)

I consider 4" version a proof-of-concept, because 5FPS screen is usable mostly for console or text-writing applications, forget about movies, games etc. You may use it as oversized MP3 player. However, 5" version with increased resolution (480x320 vs 800x480) and framerate (~5fps vs 60fps) seems to be usable enough. Since a solid powerbank seems to be enough to power the whole thing up, it seems like a nice, mobile UMPC solution for whatever needs you have.