We picked up this radio from the local antique shop for just £3. The internals don't work anymore, though they sure do look cool.
The total cost of everything was around £60 - £70. Everything was bought new, and this speaker system only uses one speaker, so we're left with a spare.
6.5" speaker. This is the same diameter as the original, so it could use the existing clasps.
180w Amplifer. This is one of those dirt-cheap ebay ones. I'm sure it won't actually pump out 180 watts (per channel), but for this kind of basic system, it's fine.
12V SLA battery. a 1.3Ah battery from maplin. It's small and cheap. 1.3Ah may not be enough though, we'll see.
Cigarette lighter. A standard 12v cigarette light from a car. This provides somewhere to plug the battery in to a charger.
Fancy Missile-style switch. This switch will be the main power isolator for the battery.
Cables: Short length of speaker cable, mono 3.5mm couple and 2 3.5mm jack-jack cables.
These are the main things. Not included about is bolts, washers, screws, or the scrap bits of wood.