This is an interesting comment, please educate me by explaining what you mean by the body (possibly) being a good enclosure "as is".
Graham,
How long have you got?
The subject is quite a complex one and a lot of people charge a lot of money to solve the problems.
I only learned the basics in my sound career and let the speaker professionals supply me with suitable products.
But the basic fact of life of loudspeaker operation is that as much sound comes out of the back of a speaker cone as from the front and the sound from the back can be out of phase with the sound from the front. If the sound from the back finds a way round to the front, the out of phase pressure waves will tend to cancel out the sound pressure waves from the front. In reality this means that an non-baffled speaker will give a poor apparent frequency response, particularly at the bass end since lower frequency sound is omni-directional and gets round to the front much easier that more directi0nal higher frequencies.
People have tried to design enclosures which attempted to avoid this - in effect to provide an infinite baffle to sound from the rear. I do remember in the 1960s when changing from coal fires to other forms of heating that a pretty good infinite baffle could be provided by fitting an airtight baffle over a redundant fireplace and putting a speaker in this baffle. The sound from the rear went up the chimney and never found its way round to the front of the speaker - at all frequencies. A colleague of mine living in the first floor of a tenement flat in Glasgow did this using a quite cheap, small speaker and the results were amazing.
But for other speaker enclosures, designers either provided some form of complex routing of the rearward sound to affect the wavelengths and/or absorb unwanted frequencies such that it augmented the sound from the front. Other designers provided enclosures which were effectively a sealed box around the back of the speaker to stop anything getting out. A problem with a sealed enclosure is that the pressure in the enclosure can affect the operation of the speaker cone - usually attenuating the output and particularly the bass frequencies. So a lot of design effo rt has to go into the treatment of the enclosure so that any attentuation is controlled to balance the frequency response of the speaker.
I look at some of the speakers with enclosures that are supplied for DCC these days and I don't see much evidence of sophisticated design. I just took an enclosure off a Loksound speaker the other day and it was just an airtight cover to attenuate sound waves from the back but, with not much volume of air, there would probably be a fair bit of back pressure on the cone. It will improve the output of the bare speaker but maybe not a lot.
In the Dapol 08, the speaker appears to be mounted with the cone facing out through the top of the hood and the rear facing into the body of the locomotive, giving a form of baffle. If the body is effectively sealed from the outside when it is in place on the chassis, then the relatively large volume of air in the body would not provide the same attenuation as a much smaller enclosure on the back of the speaker. Even if there are a fair number of apertures between the body and the chassis, there may still be sufficient attenuation of sound coming out via them that the sound from the speaker is still good. If a speaker and enclosure is fitted to the Dapol 08 it would be interesting to see if it was an improvement on the supplied bare speaker.
In the case of the Loksound speaker I referred to above, it will be fitted in the hood of an S scale SW1 switcher and I intend to build a new baffle to go in the hood of the loco which will more than double the capacity of the enclosure and should improve the output of it. Effectively, the top 1/4 of the length of the hood will be the enclosure and the speaker will point down over the front bogie.
Sorry for the length of the message but it is a pretty big subject and I have barely skimmed the surface with what experience I got over the years. My interest was sound recording and editing, both analogue and digital, and I got good speakers from people who knew what they were doing.
Jim.