Say the boundary in operational terms
Name the contact, behavior, topic, access, or commitment you will and will not participate in. For a message about how to ask someone to be your emergency contact, use this factual base: state the item, task, route, date, duration, responsibility, and any expense or risk involved.
Define any necessary exception
If an exception is necessary, give it one purpose, one channel, and a clear limit. Make a direct request, offer the information needed to decide, and confirm the return or completion plan. Make the immediate next step clear enough that the recipient can answer without reconstructing the entire history.
Respond to escalation safely
Do not call the favor small when it costs time, money, access, or professional reputation. Do not ask someone to break a rule, share credentials, sign falsely, transport unsafe items, or assume legal responsibility unknowingly. You do not need the other person to agree with a boundary for you to follow it consistently.