A few words about the (messy) insides.
Initially I started with the idea of planning and placing every bit in 3D software. Very soon that transformed into hot-gluing things in place which then turned into "just cram it all inside and hope to be able to close it". The downside of that (besides the obvious one of looking ugly and hard to repair) is that after mounting most of it inside I was too lazy to print out the new case and move everything when I had to make some changes so the slot for the power switch and battery charger are made by cutting and melting the parts of the case (the power switch looks ok, but the battery charger port leaves a lot to be desired :).
So, how is it all connected inside? We have a battery (some 1300mAh LiPo which I had from another project) connected to the standard AliExpress charger/protection (newer versions come with USB-C connector but this one still uses the micro USB). The charger output goes, through a power switch, to the boost DC/DC converter which steps up the voltage to 5V to power the pi. The motor is also powered from that 5V line and the sensors are connected to the 3.3V line from the Pi.
The battery charger has two indication LEDs on the side but those wouldn't be visible inside the case so I moved them next to the USB connector and, of course, hotglued them there. Hot glue proved to be a bad choice for that as, since there is mechanical strain from the charging cable and heat from the charging process it didn't hold well. But then I used a trick with super glue and baking soda which a good friend recently showed me (@izumitelj): when you need to fill some space and don't need/want to use epoxy, just mix the baking soda and super glue (it cures really fast so in this case it's best to just put a bit of glue and carefully add baking soda and repeat).
I didn't had any fitting ADCs so I don't have the battery voltage measurement (for now).
As for the straps, at some point I got a number of wrist straps for my smart watch so I went with that.