I would start by measuring the oxygen tank and figure out what size that pocket needs to be. Then think about the size you want the other pocket for wallet, keys, tubing, etc. You might want to consider making a cross-body bag (maybe called a messenger bag?) so that she doesn't have to keep a hand under the pouch to support the tank. Try googling "messenger bag" for the general idea.
If you really don't want to reinvent the wheel, how about starting with one of the $1 grocery bags that stores sell and decorate it for her. They certainly would be strong enough if they are large enough.
Looking forward to seeing what you come up with.