Any number of bags or tank bags would probably do it. Measure the solo rack, and look for a bag with those bottom dimensions. It is generally helpful to tie it to the drivers seat back rest (if you have one) to hold it centered and in place.
I've run a number of larger (non-motorcycle) duffels on the solo rack, sideways, and the problem was the sides drooped down and rubbed the paint on my fender and bag tops, no matter how smooth they were (soft leather). So then I cut a piece of 1/2-5/8" round dowel to length, and zip tied it across the solo rack, and that held the duffel bag's ends up straight off the paint (invisible under the bag) (and also made it easier to get in and out of my saddlebags with the duffel in place).
Finally, I found a high tech dry bag from TwistedThrottle.com that holds its shape even when empty, so it goes across the solo rack fully loaded and still does not droop down and rub the paint.
Some of those hard shape cylinder/tube packs will hold their shape (not the most user friendly to load and unload).
The solo rack really is not a very efficient rack for holding much of any soft luggage with it's curved profile.
Tank bags:
http://www.motorcyclegear.com/parts/luggage/tank_bagsTail bags:
http://www.motorcyclegear.com/parts/luggage/tail_bagsTwisted Throttle:
http://www.twistedthrottle.com/shop-by-product/luggage-racksThese things present a security problem with anything valuable you may keep in them (they come right off and walk away).
