Back Horn Speaker

This is a from scratch remodel of guppyk's design.

I am not an audio engineer. These sound fine to me, but I rarely use them and consider them mostly decoration.

# Changes:

* No diamond pattern on the skin
* Replaced nuts with heat-set inserts
* Integrated legs (shorter than original to fit Prusa Mini build plate)
* Recessed speaker for a smooth transition

# Speakers:

It is designed around these specific speakers that I already had. Fusion 360 files are provided for easily adapting the design for different speakers.

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