On Fri, 06 Sep 2019 15:30:48 -0700, Joerg
wrote:

Just received a new rear wheel. Despite being 700c and for narrow tires
the rim is drilled for a Schrader valve. Couldn't believe it. Harumph,
grumble. Why on earth ... well, it is what it is.

I don't want to wait for some fancy adapter but I do not mind making
just about anything in the garage with hand tools. My special thick
tubes have Presta valves.

What should I use? Piece of plastic? Aluminum? Glue it in place?
Anything needed on the outside except maybe a washer?

You can buy, or even make, a bushing to use a presta valve tube in a
schrader rim.

Try google.
--
cheers,

John B.