You would probably be best served with a front ported or sealed speaker. When speakers are that compact you have to give up low frequency response, sensitivity or some of both. Most small bookshelf speakers make the greatest compromise to lower sensitivity.
I assume you want very high quality speakers to match your amplifier(s). One very nice compact speaker is the
Salk Sound WOW1 mini-monitor. It is front ported and has a Seas Excel W12 woofer and a Hiquphon OW1 tweeter. The height is 10-3/4" (27cm) and depth is 9" (23cm), which is a bit deeper than your 20cm spec.
The sensitivity is quite low, 83dB, so it would depend on the loudness level you would listen at and the size of the space you are trying to fill.
I assume the binding posts are installed in a flat plate and stick out even further, but because Salk Sound can do custom work, you might be able to specify either a recessed binding post cup or the HWD dimensions could be altered to give the same interior volume, or perhaps both.
Steve