My friend had double knee surgery and asked me to make a bag that was big enough to hold her phone, tablet and water bottle, that would hang on her walker.  I jumped to the task.  I put in a couple of side pockets for pens, etc. There is a stable bottom to not fold in on itself.  Velcro attachments to go around the walker handle.

Those wonderful thinkers out there also thought this would work well for a stroller, for a bike and for a lawn chair so items don't have to sit on the wet/damp ground.

I can make from fabrics I have or something you choose.