I have a few Polaroids, and the little strip you are describing is both a dust cover and a light blocker.
A quick patch is to use a strip of black cardboard, the kind used for posters, or a strip cut from black plastic food packaging, like the trays from take-out chicken.
Use a strip of black electrical tape as a hinge.
Let the new strip hang in front of the film exit slot.
If you let the electrical tape extend over the very top edges of the strip, it will stay closed and only bend out when the print ejects.
Best wishes and have fun with this wonderful vintage camera.