You could still use this blueprint 
State Notifications & Actions. You would use numeric state above option 8 and then put in your watt setting say 60 watts and then in the Time Delay set your time to 30+ (whatever you like). Then when it is above 60 watts for 30 min it sends you the notification. It more built for what your trying to do.
You have more option to do like create action buttons so if your not home you could then send a notification to everyone to close the freezer door 
If you change your mind later and use a contact sensor all you have to do is change the trigger settings.
Blacky 