If you don't mind having the backpack laying down, you could use the grab bar and some rokstraps. You can also secure the side-to-side action with the under-seat hooks and a single rokstrap. (They're sold in pairs at different lengths)
I strap my backpack to the back of the seat using the underseat swingout hooks and the rear grab bar leaving the bottom of the backrest forward and it acts just like a backrest and carry luggage all in one. It worked great. Just an idea for ya.