I now think that the space between the keys is enough. Now I looked more closely at the form of the keys. It can be seen in the pictures of the campaign that you chose a "curvy surface", the keys are "curvy comfort keys".
I tried a lot of small keyboards (Psion 5mx, HP 100lx, N900, some small bluetooth keyboards). The best form in my opinion is a flat key, or even slightly curved concave (like the Psion 5MX). Maybe you can rethink this design? It might even make the device a bit thinner.