I haven't tried to cram the whole thing shut yet, but I'll have to soon.  The keyboard/input and power stuff is all crammed in there together with the ribbon cable for the display.  It's been getting some selective hot-glue to keep things organized.  I turned it on with battery only and it shut off after a min.  I'm not sure yet if it was more dead than I thought, maybe I accidentally bumped the power button, or ???.  Anyway, I plugged in usb power to charge it and turned it back on to make sure the screen was good and run updates.  That was all good.

Next steps are:

• Solder power wires to the pi pico
• Solder two resistors to the pico for to connect the battery to the analog input for checking voltage
• Wire the pi pico to the pi 3 for data comms... I need to read a little about which pins I want to use. 

Regarding data between the pi pico and the pi 3 - previously I'd just plugged it in with usb and used usb serial for all the data passed from the pico to the big pi, but I'm going to try using the gpio pins this time... not sure if I'll use spi, cc, or uart.  I need to read a little about which of them I can set up the pi3 to use with interrupts on communication so I don't have the cpu spinning on it checking.