Toller wrote: have a Moen single-handle kitchen faucet installed in a corian
sink. 7200
model series.
It is no long secured properly and wobbles around.

The installation instructions are simple enough... "Center deck gasket and
faucet on sink. Tighten mounting nuts from below deck."

I expect my mounting nut needs to be tightened and maybe the deck gasket
needs to be replaced.
The problem is access; I can just barely touch the nut, there is no way I
could get a wrench on it to tighten it. (yes, it is loose)
Presumably there is a special tool for this, but what? Is it cheaper to
hire a plumber than to buy the special tool?


You need a "basin wrench". Ace hardware will have one. Tom
Work at your leisure!