I've been using & elaborating on the "roll out" concept from Jeff Smiley's Balanced Embouchure for YEARS, to play well below "fundamental C" on the horn.
Not only is this useful for opening the low register, there are a number of benefits to roll out practice:
Sound like fun?
I've got a free 90 minute class published, watch it here on the High Range Magic "How Low Can You Go" Training Page.
If you want to watch the trailer to get an idea about the class here's the "How Low Can You Go" trailer:
Wanna give it a shot? Head over and watch the full class here