Looks like this. Hole is around 3.5x1.5cm and 4.5cm deep. Comes with a clip on cover like this that I would cut to put the junction box into. If it all can fit.
The bar end junction box comes with a plate for storing it in downtubes with holes in frame like this.
https://si.shimano.com/pdfs/um/UM-71J0A-003-00-ENG.pdf
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Looks like this. Hole is around 3.5x1.5cm and 4.5cm deep.
Comes with a clip on cover like this that I would cut to put the junction box into. If it all can fit.