I'd say DHA VT1. Might be slightly bigger footprint than you're after but it's WELL worth the size. They're the canine's testicles!
I think they a use 500mA supply, but the 1spot delivers 1700mA, so that shouldn't be a problem.
Newer ones use centre negative, but the older ones use centre positive, this one should see you right...
http://www.davehallamps.co.uk/page55aa.html