To make a gun strap,what I would do is.....the simplified version to get you started.
1.Oped a cube primitive and reduce one of the sides to flatten it.
2. In the vox menu ,drag the object into the spline folder ( i think you grab it via the little divide symbol to the left of the plus sign.)
3 A window should open up with some sliders in it ,there you can adjust your spline. When you ok it, this spline is then in your folder.
4 Make some curves then you can select the spline you made.
5. Do not make your initial spline piece to poly heavy as when it repeats you will get bogged down. Sometime surface objects fail to work, though voxels seem quite reliable. You just need to play around for a bit to learn what works.