It's easier done than said
Is the box big enough? I don't know, I can't see it. Could I make it work? Yes, but that's what I do.
Run a two wire cable from the ceiling box to the switch box.
At the ceiling box, attach the white wire to the blacks.This sends the power down to the switch.
Install a short pigtail on the whites, It may already be present from the old fixture. This is your neutral connection for the light.
The black wire from the sw box goes to the hot wire/terminal of your fixture.
At the sw box, two wires, two terminals on the switch.
PS. Identify the white hots with black tape.
That is how it's done in the US. Is Canada different?????