The cheapest option
Just use a duffel bag and a bungee net, but...
...if you have a little money you might consider an Eclipse Rumble Pack tail bag. It's 12x11x7" high (expandable to 12") and fits nicely on the rear portion of the 250 seat. It has an adjustable bungee cord attach system and the four hooks fit perfectly on the 250 bike hooks. Costs about $65.
Eclipse makes another nice one called The Fast Pack, and Tour Master, Ocelot and Lockhart make some good ones too.
Or you can make one yourself using a soft-sided cooler
It's an Eddie Bauer 18-can cooler from Target; regularly ~$30. Buy a $5 grommet kit at Wal-Mart (camping section) and install four grommets in the bottom four corners. Kit includes all you need; you basically punch a hole, insert top and bottom grommet pieces, and hammer them together; pretty easy. Then, use some small bungee cords (about 8 inches long and 1/4" in diameter) as follows: fold in half, stick loop through grommet (loop to the outside) and connect them in two pairs; one pair in front, one pair in rear. Hook them together with a short segment of coat hanger in the middle (if they're not quite long enough without it) and crimp so they won't come apart.
Payne has inspired some members to create a lockable, waterproof trunk for occasional runs to the grocery store and for storing helmet and cycling gear when hiking, picnicing, etc. Pretty simple. Cost a total of about $25.
It's a 56 liter Rubbermaid clone attached to the passenger seat, with bungees mounted inside the trunk and with U-bolts holding onto the grab bar for extra security. Wrap the U-bolts with electrical tape and put cloth patches on the trunk where it contacts the grab bar to keep either the U-bolts or trunk from scratching the grab bar.
This takes 2 minutes to put on or take off the bike. Attach the U-bolts, hook up the bungees, and away you go. It's got combination luggage locks that keep casual meth-heads out. A determined thief wouldn't have any problem just cutting into the side of the thing with a big knife.
On a test run to a distant grocery store the trunk stayed very secure, even in a downpour on the interstate. The paper grocery bags were completely dry. This thing doesn't have gaskets, so in case of a prolonged downpour, things might get a bit damp.
The Action Packer should be available in many hardware or other building/contractor supply stores. Here is a list of known suppliers, as of early Fall, 2006:
A rack on the back can make organizing a little easier. You might want to consider the Ventura rack.