The optical design will be the most difficult part, but I think I found a solution for this.
Follow the project logs to see the updates. The costs of the prototype amounted to 70Euros.
Video of the working device
Link to the video and Link to the video in German
One of the Prototypes
Hello, Alain! Since 2007, the bluetooth data transfer code has changed in the multimeter. Could you help to disassemble it under the sketch?