Let me suggest that you bypass the window idea. You need a more focused light source than ambient outdoor light. The frosted glass idea is good, but you don't need to build a special box to support it. Two stacks of books three or four inches high with a gap between stacks will allow you to lay the glass across the gap. For a light source you could use a white frosted or clear C7 Christmas tree light (still in its string, if you like). With the stamp on the glass, focus on the stamp so it fills the screen, and position the light below it. That should do it.
If you don't have any of the supplementary lenses for the Powershot, they are cheap and you might consider getting a set online (e.g.
ebay) so you can do good closeups. Chances are, once you get the technique down, this won't be the only stamp you will ever want to photograph. Scanning is easier, of course, but not for everything.