How firmly a haptic device, such as a smartwatch, is coupled to the body can change how its haptic effects are perceived. However, hapticians often rely on vague subjective coupling characteristics such as "strapped snugly" or "tight yet comfortable". Achieving consistent strap tightness across body sites and between participants can be challenging, since even if strap tension is consistent, differences in limb circumference alter the resulting normal force under the haptic actuator in potentially unintuitive ways. Furthermore, when participants must attach the devices on their own, e.g., during a longitudinal in-the-wild study, they may not use the same tightness each day without guidance. We present HaptiStrap, a low-cost, easily fabricated tool, as a contribution towards a standard method for ensuring that wearable haptic studies do better than vague and subjective "tight yet comfortable" guidelines.