Solar powered sound system.

[Surtsey]

Grew up near the Grand Union Canal in Croxley Green so I know well what a narrowboat is. 😉
A single 15" horn might not be the best option for playing very low and loud so perhaps consider ported, when 360l becomes quite a large enclosure, possibly for a pair of 18s bridged to give you 4 ohms. I feel there is often no replacement for displacement! As many have suggested, you will have to compromise between low and loud!

I have used these guys almost exclusively for many years. Very keen prices!

Blue Arran

Croxley Green where the Grand Union meets the Gade. I lived nearer the Colne.

I agree on compromising between low and loud. When toying with a 'picnic system' I was amazed how much power I saved by not amplifying signals below 50Hz. 600w in amps is closer 1Kw in power consumption, add exterior lighting, and it's a lot for a solar-powered system. Some get too caught up in specs.
I've never been to Glastonbury but I scroll through my experiences of outdoor sound systems and ask: did they sound good? Fairgrounds pump out a seriously amount of volume with acceptable bass although the system had to be loud enough to compete with the adjacent ride and the sound of the diesel generators. Notting Hill Carnival is the gold standard when it comes to assessing the merits of exterior sound systems.