Looks like too much work. Why do all that when the company makes a vinyl product?
"The Window/Door Cap blocks water from getting behind siding and causing moisture damage.
Protects water infiltration around windows and doors.
Designed to direct moisture away from the from the siding.
Made with heavy duty vinyl to last for decades."
I have 2" drainage plane who cares if it leaks? that why i have house wrap and flashing.