...also, a while ago I saw AA to D cell form factor converters of 3 different types on eBay. The most common one accepting 1 AA cell is easy to find, the less common 3 AA cell holder may not fit all kind of electronics (MagLite is OK, though) and usually expensive, and once I've stumbled upon the 2 AA cell holder model - less effective, but should be more compatible with tight battery compartments. Just google it if interested.

By the way, I have also standardized on AA cells. And I don't see any problems improvising a D cell out of 3 of them using some duct tape and parts from a dead D-Cell, or even just aluminum foil instead.