Building QSpeakers:

 Pre requisites:

  Install Qt5 (and QtCharts) library.
  Don't forget to install the development header files too.

 Compilation:

  You can use both command-line compilation or QtCreator.
  See below:

 In short, for Qt5 on Debian Buster, this should do the trick:
  $ sudo apt-get install qt5-default qttools5-dev-tools
  $ cd qspeakers
  $ qmake -config release && make && sudo make install

Using QSpeakers:

  Start it by your window manager's menu, or /usr/local/bin/qspeakers from
  a shell. Choose a loudspeaker, or create a new one you want to build an
  enclosure for. Change the box parameters or click 'Optimize'.

Contributing:

  If you want, you can read TODO and start contribute, or write litterals translations in your own language.

Thanks:
Thanks to Upacesky for his drawing: https://openclipart.org/detail/202508/

Happy DIY!
