You should design some where the main design elements are not in the centre, as that's where most would put the keyboard.  I like ones where the design elements are biased to the side(s) so you can still see them when a keyboard in in place.  A good example is the soya milk deskmat, or the something like the CosmiCats deskmats... or GMK Agent-01  Ideally though, as most may also use a seperate numpad on the right, a bias to the left would be good too.