Also think about it this way, if she wears a motorcycle jacket with armor in back, you don't really need any type of backrest for a tail trunk since it will hit the armor anyway. Bemster has a tail truck for under $100, I think, that includes a backrest and you can remove it. You would need something like SR357 if you plan to carry anything in it.
Current Bike> '14 FJR ES, '17 Versys-X 300 ABS
Past Bikes > '09 Ninja 250r, '00 XT225 (blew piston), '03 Goldwing, '07 DR650, '07 XT225, '12 NC700x