A bivy for rain protection (a splash bivy) is not necessary for a solo person under a two person tarp. In my experience, I am colder with this type of bivy than without because it compresses the down around my butt when I sleep on my side. If you are worried about mosquitoes, you can get a Sea to Summit nano net which is very light and gives you a relatively large amount of livable space. It’s also quite fragile. Don’t let any velcro get near it.