I went through this same dilemma not too long ago.  I used to always use a back pack but over time I got really annoyed at the added bulk on my back.  The weight on my shoulders and the heat generated on my back became unbearable.  I looked around for a sissy bar bag but the only stuff I could find were gigantic weekend bags when all I needed to move around was my laptop (sub-laptop actually), some papers, and my binder.  A messenger bag was perfect for me but there weren't any that could attach.

My solution was to take a Riggs PAK mail bag (it's not stiff like the picture implies, which pissed me off):
http://motorcycle-luggage.com/Merchant2/merchant.mvc?Screen=PROD&Product_Code...
 

And mod it with straps to connect it to the sissy bar.


There are two horizontal straps and one vertical.

Glad to help.  I'll post some better pics later.  All I have at work is my crappy camera phone.  I'll also post up what it what it looks like on the sissy bar.  It holds it on great and has never given me any problems in almost a year of use.  I needed to make some modifications to the top strap buckle though because it takes a lot of the force when mounted.  I'll do my best to document it.                              

A needle and thread.


I did it in a square with an X-in-the-center pattern

______
|\      /|
| \    / |
|  \ /   |
|  / \   |
|/___\ |

(you get the idea)

Once I get some better pics you'll get a better idea

Understand it Todd, thanks!

A sheet of aluminum would fix the floppies.

of course their bag is stuffed full of peanuts.

I've always been backpack.  I hear u guys wearing they. Out of 15yrs I've been using a pack I've never wore it. I have a $22 pack from Wal-Mart. It's a East Sport brand. It's 20"h x  13"w x 10"d approx. It has a stiff/hard base and back which makes it stand. It has wheels and telascopic handle to wheel it around when off the bihe or wear it on your back with the backpack straps. It has a section on the back that slides down over the back rest. If 2 are riding turn it to the back. If just you turn it onto the seat and it makes a great back rest for you on the long trips like I did for the 340mi trip to Buffalo, in which I had packed a weeks worth of clothes (roll them tight). It even has 2 side mesh pockets, 2 small outside pockets and a drink bottle holder. It's designed that I even have 2 12"x18" flags on it (1 US and 1 POW). You can but don"t have to use the straps to hold it on because its tight on the back rest, if you do just loop them over the turn signals. You can carry it by the handle on top also. I use a luggage lock through the 2 zippers on the main compartment to secure the contents like my fullface, leather, sweat shirt' raingear and what ever else. I carry a drink cooler in it alot on rides. lol

I just got a Marsee tank bag from www.newenough.com  http://www.newenoughhp.com/closeouts/luggage/marsee/15_liter_tear_drop_quick_...





And i freakin love it.     I found some old laptop packing material and carved out little foam niches for my gps, phone, and camera today.    To give you an idea of size, it's sitting on a c50, and i believe i could just fit a 6 pack of 20 soda bottles in it.

It has 2 ports into the underside from the clear plastic top for powering with a powerlet or similar gadjet.   The big inside part has a good sized zippered pocket, and some stretchy cargo netting.    base is a map container, big part zips off and on in about 5 seconds.     Having the speedo covered up kinda sucks...but the gps works fine to keep me aware.... do kinda miss the idiot lights though.     might have to re-route those.    If nothing else its made me even more anal about checking the fluids etc...

I used to use a backpack but the paperwork gets heavy when your riding a hour and 15 minutes to work and starts to hurt the shoulders. I have tried a duffle bag with a luggage net over it but that isn't easy to get to either. I am looking to get some saddle bags but I am not sure that they are going to be big enough.