Thanks, all. I've identified a couple of workable solutions thanks to
the replies.

Easiest is a Leviton Toggle Plastic Adapter Plate -- name may vary,
many sources, price as low as $3 but shipping always costly for a
single small item, so I've put

http://www.amazon.com/Leviton-80700-.../dp/B003F77RRO

in my shopping cart. I realized I'm not certain whether the color is
Light Almond or Ivory, but I'll probably have a chance to figure that
out before I get $25 worth of stuff in my cart. Wrong color wouldn't
really matter anyway for a small plug in a poorly lit location.

trader4 pointed me to a source for the complete plate. Thanks -- yes,
I googled it, but I obviously didn't have the right search terms ...
nowadays the best answer to a question is often just "here's the
search terms you need". I think the filler/adapter will be easier and
cheaper in this case, but knowing where to find a wider variety of
plates is a Good Thing.

Perce, I actually built a plate from those sectionals a few years ago,
but I couldn't find them when I was looking. Search terms again --
when I used "sectional" in my search, I found them. I probably got it
at Lowe's, and I'm pretty sure they don't carry it any more. Looks
like I don't need it this time, but still Good To Know.

And yeah, I should locate a Real electrical supply house. But this is
in another town and she may be moving soon anyway. And in my town,
most of the old time electric supply houses have closed. Must be one
left somewhere ...

Edward