Unfortunately, the Gopro is very limited when it comes to close-up shots. By simply mounting a magnifying lens in front of the camera, it can focus on short distances. With the lens that I used (it came from an old slide projector) my Gopro can focus now on distances between 7cm and 9cm. However, if you build that construction, the lens that you'll use will be certainly a different one than the one I found in my lens collection, so your area of focus will also be different than mine. You might also have to change the 3D file and adapt it to the shape of your lens.