I'm not an expert seamstress. I got a D in sewing. I don't have a sewing machine. But I don't let that stop me from sewing things.
To add this mesh I bought a mesh duffel bag for $5. I'm lucky they had one red duffel bag among all the ugly white ones. Buying a duffel bag was cheaper than buying the mesh as raw fabric.
I split the bag lengthwise. I threaded elastic (not visible in picture) through the drawstring area so that around the top it will be stretchy. Then I sewed it down the sides of the backpack and along the bottom. There's still quite a bit of fabric left over because I had to also cut off the bottom of the duffel bag. It was too long. I could make other things with the extra.
Now I can stuff extra things into my backpack.
My backpack is kind of small. It's intended for weekend trips and it's not really big enough to take everything you need even for the weekend. I usually end up stowing my kitchen in a bag attached to the outside. With the mesh I will have an easier time stowing things that don't fit inside the bag.