I can't stand a sweaty back when I bike, which is why I switched to panniers ~10 years ago. It's also much easier on the back in my opinion. You can get by making this pannier with a waterproof/coated fabric (I'm using 500D Cordura), some easy to find hardware, and a home sewing machine. Other things come in handy, but you can make it work with the bare minimum.
Look through the pictures and skim the words a bit before you start. If you get stuck or something isn't clear, drop me a line and I'll try and help out as best I can.
I'm adding some optional materials/tools to either make things easier on you, or give you the option to add some accessories to your pannier. I tried to group together items from the same carrier so you can cut down on shipping cost, assuming you don't live near a decent fabric store.
Materials you definitely need: