furnace vent cap

The vent for my furnace does not have a cap and I can't find one to fit it, or at least I didn't find one at Lowes and can't find one on-line. It's one of those older type vents that is rectangular with rounded ends, approximately 12" x 4". The canopy of a big oak tree is over that part of the roof and I'm concerned that over time the vent will get clogged with leaves and twigs.
I haven't been into an HVAC store yet, but the ones on-line do not show anything that is that shape. Does anyone know where to get one? Would it be ok if I just shaped a mesh screen and fit it overtop?
TIA
Todd
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Water going down the stack,,, probably not a good idea for the long run.
Buy a piece of sheet metal 20 x 10 and bend it in a half round and screw the sides into the pipe making a cap that will look like a "T" sort of. Just my idea.
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This is Turtle.
Go to your local hardware store and ask for a 4" hot water Tank vent pipe cap.
TURTLE
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