I put a micro fiber towel under mine to protect the paint. If not you run the risk of it rubbing and making things shiny. I think you mentioned a back rest and I use a cheap backpack that straps onto the backrest on mine. I think it's around 20 bucks.

 

Its super easy to strap a bag to a sissy bar. gotta be less than $50-$100 depending on the bar.

 

Here's a pic of my set up

 

Here is one way and you can bring as much stuff as you want.

But seriously, the garbage bag, sea bag works just fine, as long as you keep it down to a few items.

 

I've used the duffle and cargo net many times. It always works and is a great cheap option. I have a Biker's friend bag that I'm selling in the classifieds only because I got some large hard saddlebags now. Its another great option that holds its shape, looks great, and can easily hold a weeks worth of gear depending on how you pack.

If you can find a set of cheap saddlebags and get a hold of a set of Easy Brackets it also makes for a great inexpensive quick detach setup.