Sewing question for car seat organizer?

I`m making a car seat organizer from exemple found on-line. My only issue is that the finished product turns out from the weight of the items the kids put in their respective organizer. I currently have two ties to attached them to the headrest and one elastic about 4 inches from the bottom to hold it in place.

I thought of putting another elastic at the top but we found that it gave us back ache. I`ve done another one with a thicker material doubled and sown across the top to help it keep it`s shape and I`ve mitered the corner instead of the rounded corner I orginally did. This design is better, holds better but still turns towards the kids with heavier container (for exemple a sippy cup or juice bottle).

I`m not sure what else I could try and would welcome ideas. Here is a pattern similar to mine.

car seat organizer free pattern picture.