Welcome aboard! It's always great to welcome new members on here! You found exactly the right thread. I got two TVC AT-RT Drivers that are missing the backpack, too. And I got one complete figure. I'm going to make a mold of it to cast the backpack in resin to complete the other two. If a resin cast is an option for you, just send me a PM.
If you rather look for an original part, I wish you good luck and hope you find one.
Best regards from Germany!
- Philipp