You can buy $5 bottle cages and $100 bottle cages. Same with racks and supports....prices vary partly on how well they are made and the materials used. Sometimes the "coolness factor" comes into play in price as well.
I originally had a bag saddlebag support that was cheaper and simpler...and it scratched my frame paint very badly.It turned out to be worse than just wasted money.
Yep, you can roll up an extra shirt or a lightweight windbreaker and tuck it under the strap on top of the bag.That small bag you have says there is room for a rain jacket. That doesn't look possible! (oh, I see, on the outside!)