We have the MCS fuel gauge, top entry onto the tank. It does rely on not losing any air pressure ever, but of course it does, slightly. I have to pump it up about once a year but it only takes a few minutes.
In the OP’s case I have no idea what sort of input the Cerbo needs. The sensor for the MCS device is a pressure sensor which is a bridge, so the output would be very small and some amplification would likely be needed to get it into a range suitable for the Cerbo. Or just steal the principle and find a low pressure sensor with a suitable output, the rest is just plumbing.
https://www.mcsboatproducts.co.uk/our-products/fuel
Edit: looks like they’ve changed the design. Ours had a Schroeder air valve to top the pressure up in the down-pointing pipe, and it was much narrower than the type shown.