These stainless steel Maillon Rapide connectors are a good way to attach your reserve to your reserve bridle, your bridle to your harness, and are especially handy when you've landed in a tree and need to disconnect your harness from your reserve. Gently tighten and loosen with a wrench or multi-tool. A gate-locking measure (tape or heat-shrink) should also be in place.
6mm is the most common size and is plenty strong. 8mm is super strong and probably overkill, but available if needed.