Windshield cleaner: I use Plexus or Prist. Any shop towels will work, just make sure they're soft and won't scratch.
M20J towbar: I might be willing to send you mine if I can score one of the Redline ones. Anyone know if Redline lets you pick it up? I'm close to SEE...
Fuel tester: I have the original that came with mine. But I have a GATS jar so I don't need it. PM me if you'd like to work something out.
Stratus vs. Sentry: I vote Sentry. Weighs less (stays on your window better), has a CO monitor. I've owned both, and if I was going to buy one again I'd get a Sentry.
Packing list: Definitely oil. Oil funnel. Screwdriver (better to get the set with both Philips and flat-head). Batteries for headsets. Flashlights, head lamps. Air pressure checker. If you don't have LED lights, replacement bulbs for your nav lights (keep in a padded box). Sick bags for pax, and a pee-pee solution. Invest in some airframe screws (Aircraft Spruce sells screw sets) in case you lose some. I checked the plane's belly in my first couple months and ownership and found three missing!
Control wheel locks: After calling the company directly (referred by Spruce) to help me pick the right one, I got this, and it works great: https://www.aircraftspruce.com/catalog/pspages/controlsaver13-15505.php?clickkey=4469
Anything else?: Aircraft cover, if you don't have one already. Amount of coverage depending on the weather and how long you expect to leave the plane out during trips. BatteryMinder.