Thats one of the reasons when I bought my Corbin seat I bought the backrest with the opening in the middle. I regularly have to tie things on the pass seat and having the backrest with the hole in the center makes if very convenient to loop bungees to. The other thing I Have found that really works well is the elastic cargo net. On smaller things it works perfectly when fastened to the passenger hand holds and the over the backrest and then latches on to the back of the passenger hand holds. Anything that is on the seat it holds securely and I have no fear of loosing it. If you could get a bigger one I think it may work well.
I have had some pretty big items on my bike without problems with the cargo net over the top and if the package needed to be a bit more secure then bungees through the back rest. At first I bought mine as a whim and didn't see how great it would work but after using it a bit I wouldn't be without it. Mine stores small, holds a lot securely, lightweight, easy to use it had plastic hooks on it that I took off since I didnt want to scratch the paint. That is also another great thing you dont have to worry about scratching the paint.
The passenger hand holds make the perfect thing to attach the bungee to. It catches on the ends easily and securely and looped over the seat and backrest holds anything on the seat securely.
https://www.amazon.com/Avenir-Elastic-Cargo-Net-Black/dp/B001U7SYK0https://www.organizedobie.com/categories/259/Barrier-Stretch-Netshttps://www.google.com/search?q=cargo+net&source=lnms&tbm=isch&sa=X&ved=0ahUKEwiOs8WNqZ3NAhUL2B4KHS-eA3wQ_AUICSgC&biw=1433&bih=947#tbm=isch&q=cargo+net&chips=q:cargo+net,g_1:motorcycle&imgrc=_