Leaves in cowl and heater

[gearboxx70]

My '67 burb is a Daily Driver, and is usually parked in the driveway (since I have no garage). I get all kinds of leaves and pine needles falling into the cowl and through to the heater box. So, short of fabbing a new cowl with a different type of air intake, what can I do to keep the garbage out? Can I seal it off with some kind of solid material such as plexi-glass or sheet metal and still have a working heater in the winter?