Note: even the P size will show in the shot with wide angle lenses. e.g. my sigma 10-20mm at the 10mm end.
so can you get an adaptor to fit the canon 450d? if so what size would you have to get?
What sort of filters do you want and what do you want to use them for?
Rather than getting a square Cokin, with an adaptor then fitting the filter, you'd be better off getting a circular filter to fit your lens. No adaptor needed.