Some advice I've read/been given suggests leaving the hydrometer in the beer
during the fermenting. I am using a Coopers Kit fermenter and cannot see how
I could accurately read the hydrometer without opening the lid and thus
exposing the beer to 'nasties'.

Is this advice meant for glass car-boy type fermenters, has someone come up
with a way to read through the opaque sides of a plastic fermentor, or does
the brief period the lid would be open constitute acceptable risk?

The lid of the Coopers fermentor is a clearer plastic, however the
condensation build up and the angle you'd be required to read from doesn't
make this an option (I don't think).

Many thanks, Michael Mowbray.