I too have thought about a separate amp top (in the currently-fashionable metal-clad format, so reminiscent of the 1960s Selmer "Twin Lead"), but haven't so far taken the plunge.
I have a Carvin 1x12" cabinet fitted with a Celestion Vintage 30, and I'm sure that any similar cab would handle the output of a 15w amp more than adequately. Epiphone do one which is very similar (fitted with a 70w Eminence speaker) for about £109:
http://www.guitarampkeyboard.com/en/73221It looks nice too - suggestive of traditional Vox styling without being a copy.
Hope this helps,
JN