I will be making the playable VR demo using Unreal Engine 4, just for your information. I have been developing UE4 games for two years and am experienced with VR development, my development process is fully compatible with Google Cardboard, Samsung VR, HTC Vive etc virtual headsets. If I understand correctly you only need the app to be compatible with google cardboard right? I can get it done within a day. Looking forward working with you!