One of the cheapest possibilities to build a good sounding speaker with also good efficiency and compact dimensions.
The Loudspeaker
As speaker the FE103 Sigma from Fostex is used. The driver has a diameter of 10cm and a white Membran.
The efficiency is approx. 90dB. The cost is very reasonable with approx. 50-60 US$
Resonanse 70Hz, efficiecy 90 dB

I used 19mm thick MDF plates. Within the enclosure I glued a 10mm sandwich Alu/rubber/Alu on each side, the top and the bottom plate. As damping material I used wool which mechanics use for cleaning up oil. 20 mm on each plate is enough. There are no capacitors or any other electronic parts within the signal path. If you want to reduce the higher frequencies a bit you might include the parallel connection of 0.68mH and 4.7 Ohm in series with the speaker. I used this for a while but recently switched back to the version without any electronic parts.

How does it sound?

It the fantastic location of each instrument or voice. The midrange and hight are superb and even the bass has a wide range. Especially for this small speaker it is really good. O.K you will not feel any punch in your stomache but believe me, you will be impressed.