Never ever close this area using waterproofing membranes etc.
The area is known as a closure and there are specially made up fittings you can get from the hardware or your roofing supplier.
Install poly buttons and extend the flashing so as the water blowing under the open area has to travel a longer distance and also try get passed the poly button.
In the picture the problem is aggravated as water that gets in though any access point cannot leave and thus back spills into the property and also causes rust.
This is a premier customer of ours and these luxury rooms (4 of them) cannot be booked out as this DIY attempt did not work.
“THINK LIKE WATER” is one of our sayings at ARK.
Many problems can be avoided by using the statement above