I commute on my 1198s, and took it to VIR for the AMA round. I use a cargo net secured by the rear seat. Going to/from VIR, I used my GI Duffle bag, one each, to carry my 3 days worth of stuff. Normally I lug a laptop back and forth to work, the only real trouble I had with the duffle was it was taller and as a result tougher to mount the bike. Else, it worked great. Waterproofing is cheap, one hefty cinch-sack for a total outlay of:
Hefty plastic bag: $.85
cargo net(black): $9.44
GI Duffle bag : $ -.-- N/A - just 6 years of my life.
Just about the cost of a full tank of gas.