I chose to not install the Princeton capacitance fuel probe based on my previous experience using this probe. It works fine if 100LL is used exclusively, but really suffers if you put auto fuel to it, and will die in short order if any ethanol-laced gas is used from rapid corrosion inside the probe. Additionally, this probe can only be calibrated for one type of liquid because it relies on a constant capacitance value of the liquid it is measuring. Change liquids and the capacitance changes as well, totally messing up the voltage signal output to your gauge. In essence, with 100LL a half tank reads half full, with car gas it reads 3/4 full, and with ethanol it reads full all the time. This situation was very frustrating for me because I routinely mix 100LL with car gas (including ethanol at times).
Instead of the probe, I chose to install a flow sensor and use the Dynon fuel totalizer function to keep track of fuel quantity. Dynon supplies the Flowscan "Red Cube", and it works great! I install mine right after the shutoff valve. It is inside the cockpit where it remains cool and low vibration, and it also measures all the fuel leaving the tank, rather than just the fuel entering the carb. This could prove a safety feature if the sump valve ever failed and started pouring fuel overboard; the sensor would see it and keep track of the loss.
As well as the flow sensor works, I can't help but get a little nervous as the tank gets low. I wanted a way to confirm what the Dynon was telling me when approaching the VFR fuel-reserve level. Because the rotationally-molded, cross-linked fuel tank is translucent, you can see the fuel level through the tank when the lighting is right. It's possible to lean under the panel and look at the tank to see the fuel level, but this is hard to do in flight, or when sitting next to a passenger.
My solution was to install a small backup camera system with a 4" monitor to observe the tank for me. I aimed the camera at the tank, added a few lines with a sharpie marker corresponding to the last remaining gallons, and tucked the monitor along the sidewall out of the way, but easy to see while in flight. The last piece of the system was an LED array shining into the tank. There are many styles to choose from, but I used a 24-segment LED light intended as a replacement auto "dome light". This back lighting really makes the fuel level stand out.
I wired the whole thing up to a switch so it only operated when needed and the system was complete. Total cost was under $40, and total weight is under a pound.
Low Fuel Monitoring Camera Video (YouTube)
Fuel System Diagram (pdf)
NOTE: photos link to full size image