Do the canon images show the same? I've recently found that the qhy8 sensor is 2mm larger across the diagonal than the 40D sensor. Not sure which canons you have
[edit] Found that the 350D sensor is the same size as the 40D but the 300D is bigger. The qhy8 is 28.4mm across the diagonal while the 300D is 27.26mm and the 40D and 350D are 26.68mm.
There is a 13mm difference between 2" and 2.5" but a $30 light box is really what you need.