On Wed, 06 Apr 2011 15:44:07 -0700, Joe wrote:

Take your specs to a good sheet metal shop and have them build you one.
There are nice coated sheet metal types available today that are perfect
for painting. If you get one made with these modified galvanized
materials it should last for many years.

Here (finally) is a picture of the ruined metal enclosure.
http://www.upload3r.com/serve/080411/1302291053.jpg

I was wrong on the specs - it's about 2 1/2 feet tall, about 5 feet long,
and about 2 feet wide (that's a yardstick on the ground in the pic, for
scale).

The enclosure appears to be of galvanized steel, riveted together.

What I would need, to withstand the frequent 100mph winds, is to
reinforce or brace it a bit better.

But that still leaves open the question whether this chimney enclosure is
merely aesthetic or functional.