The fuel level sight gauge starts with an aluminium tube. It's cut to length (13cm) and a slot is cut and the edges smoothed.

The medical lab glass was ordered to the correct length (15cm)
However it is a smaller diameter than the ally tube so it needs a couple of layers of heatshrink to be added to stop it rattling around.

The glass tube can now be inserted into the aluminium tube. The heatshrink makes it a snug fit.

And here's the finished article ready to be fitted to the tank. I think the clear flexi pipes on each end are a little long but they can be trimmed down when fitting.

I had considered a little "pea" float as a level indicator but cannot find a UK supplier. It would need to be tiny now anyway as the glass tube ID is about 4mm. I'll see how I get on.