I had the same problem, so I made my own,
Buy a school type wooden ruler and paint it with primer so the fuel level will show.
Drain tanks to empty and add fuel in 10 litre, or I guess 2 or 3 US gallon increments. I needed to put about 15 litres/4USG per side before the dipstick showed any fuel at all, and mate my first mark at 20 litres (3.8 US gallons per litre).
I continued in 10 litre additions, marking with a felt marker at each point, til the tanks were full.
The stick was inserted vertically against the back of the filler neck.
After a few weeks when I was sure I had an accurate dipstick, I had a friend make a copy from aluminium with markings engraved.
Now I am confident as I know exactly what fuel I have when I start up.
Remember to subtract the unusable fuel to get fuel available. I use 6 litres per side, the flight manual/maint manual/POH were not consistent on this so I erred on the safe side.
To get max flight fuel, remember to put in the gallon you will burn on taxi and takeoff, you will still be below MTOW when you get airborne. I find this helpful as range is a limitation with two people.
Regards
John Kempton