I use an Op/tech strap, the 'Pro Loop' model since the skinny loops for connecting to the camera drop away from the hood nicely.
I thread the 'Pro Loop' part through the Rolleiflex straplug; most will require a little pushing but it will go through.
Most often I simply connect the two short pieces of the Op/tech strap. This makes a wrist strap. I put them hand through it, twist a couple of times, and hold the camera in my right hand almost exclusively. With some levering and relaxation, I can carry a Rolleiflex all day like this.
For over-the-shoulder or -neck carrying, I throw away the rest of the Op/tech strap. The neoprene padded part doesn't work for me. But before I throw it away, I cut off the two clip connectors. I then use a 3/4inch strap, like you get for sleeping bags at a camping store. Or a thin camera strap like from older SLRs or such. Sew the Op/tech connectors onto this, using the strap's original hardware for length adjustment.
I usually put the strap across my body rather than around my neck. It's easy to push it back or slide it forward as needed, etc. And less bouncing and such.
When I am not shooting and want the camera away, I use a Lowepro Adventura 120 bag. The Roleiflex fits in snugly. room for a meter, a filter, some film. This small bag is padded and can be put in a knapsack, bike bag, etc. as needed. I prefer a 'modular' approach to camera bags.