I don't know how I missed Ted's previous thread on this...
Only question I have is do I need to alter the size at all for a 3" screen (in other words, are the D90 and the T2i similar enough in design for it to work as is?).
Basically its the same design except for small grooves and pits near the viewfinder base, which is a problem if the plastic container you will find is larger than the boundaries of the LCD screen, this aint exact science since the plastic containers you may find in your locality may vary in size.
The depth of your container would depend on the power of your magnifying glass, as its magnifying power determines its distance from the LCD screen. I found the 2.5 inch diameter glass has no distortions on the sides of the lcd image.