I bought an antique, brass, house key at an antique stall and I keep it on my keyring. It is the key to my own secret place/room. Maybe you can see if T has any odd/leftover keys that don't go to anything anymore (I have tons, long forgotten what they open) and he could give you one and it could symbolize his door? Then you would know you have access when you need it but are also a co-keeper of the door so it's not used inappropriately.