I have very clear memories from at least 45 years ago riding to school with a rain poncho on windy rainy days, the loud flapping sounds, the bike getting pulled this way and that from the sail effect--these are my personal reasons for not going the poncho route again for biking, especially not bike touring.

If you really do like using rain ponchos, I suspect the companies that have been making them for decades and decades have very efficient designs, but then again, one can only applaud doing a cheap DIY project, so have fun.