You can buy p-clamps from a hardware store for less thathan $2 total. They will have rubber lining which keeps me on the frame and keeps me from scratching. They come in different sizes so you can fit me to the size of your seat stays.

You can also get a seatpost collar adapter and other options here- https://www.bikebagshop.com/rack-accessories-c-89.html

Then there is this-http://www.amazon.com/BIKE-RACK-SUNLT-MONOSTAY-ADPTR/dp/B003MNSG8K

Get a rack, grab some used pan bags off Craigslist, and you have more options for clothing, cooking, etc.