Nice build, thanks for sharing 
. I built a similar-sized one (~10 liters, 12/15mm multiplex) two years ago, even with the same subwoofer. However, my fullrange speakers were 4" Dayton Audio RS-100-4 and I used a passive radiator instead of a port. Since the plate amp was not available back then I used the standard amp 2.1 board. Meant, I had to create my own front and back panels. Fortunately, the plate amp makes things much easier in this regard. I also have a plate amp 2.1 laying around on the shelf but have not made up my mind on the use case and design.
Regards, Sven