Short update on my progress:
To make everything smaller and more mobile, I designed a new case with a ultra slim 10000mah powerbank from TNTOR. As I didn't need the USB and Ethernet sockets I slimed down my Raspi as you can see on the pictures. The new casing is now already quite neat and slim but the elastic band with velcro fastening didn't work out as expected. It is a little bit too limp and has too less adjustability to different arm. I might have to come up with a different solution here.