The humidity in the crawl space is high during rainy season (winter) here,
and I want to set up a dehumidifier. This means I need to close the crawl
space vents temporarily. In the summer I won't need the dehumidifier and the
vents would have to be opened again.

What's a good way to close the vents that is easy to re-open? I prefer being
able to open and close them from the outside so I don't have to crawl into
the crawl space.

Some of my house vents are "sunken" similar to this pic (not my house, just
using it for convenience). Actually mine is sunken even deeper.

http://www.pugetsoundhomeinspections...143554_std.jpg

I search the web and found only one closable crawl space vent. Is there
something I can pick up at a local hardware store?

http://www.iaqsource.com/product.php...product=174938