We needed to use a wheelchair for my son's broken leg this winter. When it arrived, the first thing we noticed was the lack of carrying space, especially since we have recently used strollers, which have copious storage for the parent and child, plus extra underneath.
We pretty much immediately set to work making a wheelchair bag for the handles. My son chose some heavy canvas-weight Ikea fabric I had (stripes and a coordinating pattern).
To make the bag, we measured against the wheelchair for the width, then folded it in half and stitched the edges together. You could make it any height you want. Ours went less than halfway down the back of the chair.
Then we stitched a hem around the top after pressing it
To make the straps, which are 1" wide, I held up the bag to the back of the wheelchair and decided on the length. Then I cut them 2" wide and pressed them inwards with edges hidden. Then I stitched up and down both sides. These are attached to the bag using a box stitch in four places.
This ended up being just the right size to stash his water bottle, a tablet, and headphones.