The little I know, some touch devices can be made to work with anything (pencil, finger, gloved finger, etc.), some need human finger to make them work (the ones that you need to take gloves off to get them to do anything), can't remember reason right now.
Possible your device needs 2 inputs to do anything (light sensor detecting a shadow/darkness AND a button being activated), so - I have no idea how you need this to look when always on, so may not fit in with your objective - try experimenting with placing different materials on the button spring you need in order to lightly weigh it down and perhaps also necessary covering/shading the thing that looks like a photodiode. Whatever you use, if doesn't have to be warm-human-based to touch the spring/plastic interface, maybe can place something over it just needing be very close if it is a proximity type of button.