1) Confession. I'm not a Rollei user. As I recall the Rollei uses a horizontal piece over a post then the strap clips to the post. If not, ignore the rest of this post.
2) So consider this as an outside the box suggestion.
3)I use gripper straps that thread through a typical split ring. As an alternative I'd look at threading the web portion through the horizontal section & pull it upward. Then use a split ring to attach the free end to the section of web to the upper section.