An apparatus includes a handle assembly, a shaft assembly, an end effector, and an auto-tensioning feature. The handle assembly includes a body and a plunger. The shaft assembly includes an external sheath that is fixed to the handle body and an interior shaft that is coupled to the plunger. The interior shaft is slidable relative to the external sheath. The end effector is configured to encompass a bodily lumen and includes a flexible member extending distally from the interior shaft. A first coupling element is fixed to the distal tip of the flexible member. A second coupling element is fixed to the external sheath. The first and second coupling elements are magnetically attracted to each other and are biased toward each other such that the flexible member defines an adjustable loop. The auto-tensioning feature is configured to bias the plunger portion proximally relative to the handle body.