I chose the JBL CS742's as they are small (4"), cheap (£20/pair), have relatively high sensitivity (90dB/W at 1m, claimed) and low bottom-end response (claimed 75Hz).

This log documents the design of the enclosure to take the two speakers.

12/6: I've been playing with this bodged-together enclosure for a while now, and learned a lot. It's got a removable top and moveable back so I can change the volume easily. It's also got holes for two 20mm ports, and I've cut some 20mm tube to various lengths. The upshot is: completely sealed is best, ports do almost nothing at this size! Plus, my coaxial speakers have no bass response to speak of. Probably not surprising.

So, big 180 degree u-turn. The new plan is to use a single 4" coaxial mid-top and a 4" sub. I can use the two amplifier channel to drive each separately if I mix the left and right down to mono. A passive radiator or two will hopefully solve the problem of the port doing nothing. Parts on order, just have to wait...

19/6: Started thinking about final enclosure design. Solar panels stacked end-to-end:

...or if I turn the solar panels around to make it short and fat:

Think I prefer the latter.

6/7: Have now fitted the passive radiator and swapped out one of the coaxial speakers for a dedicated sub-mid. I've also partitioned the box into two spaces for each of the speakers. The amp is now wired as mono, with one channel driving the sub (with a low pass filter response) and the other the mid-top.

The box generally sounds pretty good. Bass is not bad, but let down by the crappy speaker I bought (it was very cheap!). I've seen one for a bit more cash with an xmax of 4mm and an Fs of 73Hz, so I might try that - plus wind the gain in the amplifier up a bit. The passive radiator works surprisingly well...

7/7: By way of some kind of semi-scientific method, I measured the response of my speakers plus a Gale Gold monitor for reference. The response of the microphone is not compensated - I reckon most some of the low-freq roll-off is due to the mic, not the speakers (sealed enclosure roll-off should be 40dB/dec and I've measured 60dB/dec). But I can at least compare them:

There's some peaking at ~200Hz (Fs of the speaker?) that I'm going to have to watch out for. Hopefully the new driver will be a bit flatter/have a lower Fs.

Just ordered the new speaker: it was a toss-up between the Peerless SLS-85S25CP04-04 and the Visaton KT 100V. I went with the Visaton after a bit of modelling in WinISD shows it's apparently better at <100Hz response in a small volume.

11/7: Got the new Visaton speaker today - so much better! Check the measurements (now normalised to the response at 500Hz)

It's about 10dB better than the JBLs and the cheapo 4" driver at low frequencies, and not far off the same response as the Gale monitor, at least down to 40-50Hz. Plus when driven hard it doesn't tear itself apart like the other one - lots of excursion.

14/7: I've cut down the prototype box to roughly the finished size, and am planning on taking it camping this weekend as a test run. Here it is: