It can be a bit difficult to find commercially available speakers which work off DC or batteries, in fact the only one I know of is the Remote Audio Speakeasy, which isn’t easy to get hold of over here, or cheap.
I normally use a dB technologies L80 if I need to use a speaker, although it’s mains powered only- it’s a 35W amp driving 4ohm speakers and loud enough for most situations. Having taken a look inside, the main transformer outputs 23V, and I’d have to use regulators to get that from my battery power.
After a bit of a rummage around drawers I remembered I had a board for a 25W Tripath 2020 Class-D amp (you can get these off ebay for about £20, and they’re rather good) which ran off 12V.
I’ve got a little Yamaha NS10MM centre speaker, which I normally use as a ‘Grot Box’ monitor which I bought from Hard-Off (yes, it’s called that, and it’s brilliant) in Japan for about £12 (it still has the label).
I soldered together a few connectors, hooked them up to a 12V Li-Ion pack (another cheapo ebay job) and, as I didn’t have a suitable case for the amp, and the speaker is likely to get chipped, put it all in a camera bag:
I also found out my passive DI works backwards, so can send balanced line out to this or hook up a wireless receiver to the bag