wvmarle, thanks for the suggestions! I was just thinking something approx. 30cm round, the beam sensors sound a bit tricky due to the need for so many so close together! The scanning laser in theory would work well but I'm not sure I've got the skills for that! Lasers worry me that I'm gonna blind myself accidentally! haha
dougp, can you think of a medium that might work better than Velostat? I've only leant that way because I could basically put the 2 layers together without having to worry about any kind of spacer and rely on the impact to change the resistance compared to the baseline.
Thanks everyone for your suggestions and help, it's really getting me thinking!