Stop water leaking through cowl seal into heater box

I have a 2000 Honda Accord and when it rains fairly hard, the rear passenger floor gets wet. The top of the floor mat, however, will be dry.

After looking under the front passenger seat while my wife drove, I observed water coming from the heater vent under the seat. Apparently, rain water enters the heater box under the panel that covers the gap between the hood and the windshield (cowl), then drains into the cabin.

After a little research, I found the cowl is seated on the windshield with a felt type material, which if it fails, water can enter the small channel beneath the cowl and flood onto a lower larger channel where the heater box resides.

I found a good youtube video of a one mechanic's solution where he fabricates a bafffle over the heater box to deflect water that leaks under the felt seal. He doesn't usually replace the felt seal which appears to be hit or miss.

Questions:
How can you ensure success if replacing the felt seal?
Any thoughts on fabricating a baffle to cover the heater box?