In article .net,
DaveC wrote:

sure. generally it's 2mm thick optical glass that's typically a 2" or
3" square piece and you'll need to cut it to fit the camera.

Seems straightforward (notice the absence of the term "simple").

if you know how to cut glass it should be fairly easy. however, opening
up the camera to replace the filter is usually not easy at all.

Can you give a URL for such a piece?

here's a few:
http://www.alpineastro.com/filters/filters.htm
http://www.controloptics.com/product_colorglassfilters.htm
http://www.edmundoptics.com/onlineca...cfm?productID=
1918

The proper thickness of the replacement glass should be...? Should it be the
same thickness as the filter removed from the imager?

it should. if it's not that close, you might lose either infinity
focus or close-up focus, but stopping down should compensate for that.
depending on the camera, the autofocus might need adjusting.

the other problem is the only way to know how thick a filter to get is
open up the camera and measure it. it varies depending on the camera,
and sometimes it varies with the *same* camera, depending on the
revision. i've also heard of using microscope slides to match the
thickness.