Waterproof cap to a backyard wall

This cinderblock / concrete combo wall was installed by the last owner. We just had the roof taken off the walkway because we wanted more light back here. Unfortunately, this exposed a 4” hole at the top of our wall between the wood frame and our neighbor’s wall (which seems to be some flimsy siding). How can we cap the wall so water doesn’t seep through our wall? Any ideas appreciated! We did have one suggestion to install a metal cap but the price was $600 😩

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