I don't understand your math. The 1mm (the wall) was there already, so why is it being counted here? Plus, multiplying by 1 doesn't do anything? Also, the 2mm extra won't be solid plastic (they'll be solid air, since that's why we're adding the extra thickness, for the room.
If anything, the extra material for the case would be the perimeter length times the perimeter wall width times the height.
The post was fixed about 30 seconds after making it - due to the *s being interpreted as italics. It is a shame there isn't a preview button when composing posts.
There's a new field in your profile called delay. It's the time delay in minutes between when you create a comment and when it becomes visible to other people. I added this so that when there are rss feeds for comments, users can, if they want, have some time to edit them before they go out in the feed. Many users edit comments after posting them, so it would be bad if the first draft always got shipped.
Delay is initially 0. The maximum effective value is 10. It only applies to comments.
If anything, the extra material for the case would be the perimeter length times the perimeter wall width times the height.