Thanks for the update and glad you aren't having much pain! Good idea to go back for advice too.
Plastic bag round arm : large, post office size elastic bands? Get a large black rubbish sack and lay on a table: arm onto sack and pull up - do you have blue tack, if so are you able to stick the bag onto the plaster with BT? then pull up the other side of the sack and again, BT. Then pull up elastic bands, one as far as the elbow level the other at the wrist?
Or ask a niehgbour? I'd help if I could reach

pity someone hasn't invented something!