You should not be even thinking about training legs with a plaster cast.
I came off a motorbike, fortunately I was wearing a leather jacket and only me knee had a surface wound. I needed a caste so the stitches would hold.
I didnt even try to raise my leg all the time I was in a caste, cause it hurt when I tried at first.
The day came to have my caste off, and I stepped outside and could not walk up a curb. My ankle ect was ok to walk, but the whole leg was just shrunk to not much.
I did squats before I had the accident and it took 4 weeks after the caste came off for me to start again with squats. I remember riding a cycle during those weeks.
It took a year for me to be satisfied that both legs looked equally strong. I had strong legs. The nurse even commented on how heavy they were to lift.