Its a lot of hassle, youd be better off replacing the centre console itself than trying to modify the current one. Normally people use double din for tv/dvds etc, but you can get 7" screens in single din that pop out or up to about 4.3" touchscreens on removable fronts. (which is what Ive done)
Due to the fact that the single din slot in a saxo slopes down if you have a head unit with a large face it fits well as it comes outwards over the top of the heater controls without interference.
If you do decide to go ahead with a double din, youll need a lot of time and money, so let us know and post pics, itll be interesting to see!