Target marker occlusion

Hi, I am working on Android application which utilizes Unity and Vuforia library for detecting markers and rendering some 3D objects on them. Now I am trying to figure out how to implement markers occlusion caused by a hand. When a hand is over a marker, 3D object should be only rendered on visible parts of the marker, not above the whole hand. Do you have some ideas how to do it?

Currently, I am putting virtual buttons in rows and columns on the marker and detecting which of them are triggered. When virtual button is triggered depth mask is displayed on that virtual button masking some part of the 3D object under the hand.

Probably, the best solution is to use OpenCV, but it costs some $.

Thanks for any help.