I just used nylon drawstring bags, adding hooks on bottom to go around seat tube, a strap at the top going around the head tube, and a strap on the side to loop over the top tube.

Just tie hooks on the drawstring ends out the bottom, loop around the seat tube and hook back into the eyelets. Costs basically nothing and works like a charm as long as you don't overstuff it (making you ride splayed knees).

ps, the trick to that is having the head tube strap on the same drawstring as on the bottom corner going around the seat post. That way you get a nice tight line on the bottom edge which keeps it from flapping around loose. the nice thing is you pull it off and it's just a bag like anyone else might carry.