I made this kind of small box for my IMU with a step down and an HC-05. So I can place them wherever I want on my tractors.
However, I encountered a problem. in this configuration the arduino seems to launch too quickly and does not give the BNO time to start. therefore he cannot find it. I therefore suggest simply adding:
void setup()
{
delay(5000); //delay for start BNO
Serial.begin(38400); // Start serial port
this does not happen when powered by USB. I suppose that the step down takes a (tiny) time to stabilize at 5 V, and that the arduino starts before unlike the BNO.