I usually carry a roll-up canvas shoulder bag or cloth hobo bag in case I want to pick up a few groceries after a ride. I keep a few basic tools in there. On longer rides I might add a spare tube, extra water bottle, snacks. After major adjustments or parts replacements I'll add a few more tools for the first break-in ride. I just roll up those bags and use a cargo net to hold 'em on the rear rack. Easier to tote with me when I'm off the bike than a conventional trunk rack.