If 20min intervals aren’t doing it for you, have a look at the following series of workouts: Pioneer, Phoenix, Gibraltar & Polar Bear.
They offer 45,75,90,105mins of rolling SST at intensity ranges of 80-85%, 85-90% and 90-95% FTP…these should help bring some hurt back in your life 
In the workout description, it says that they are appropriate for triathletes, so they may suit your TT goals.