Where can we have sex?

So I’m 18 years old and my boyfriend just turned 21 years old about a month ago so we both still live in our parent’s house. His mom is always home, my mom is always home so whenever we have sex it’s usually in the backseat of my car and I’m a bigger girl so car sex is a little tight and uncomfortable for the both of us. Money is tight so we can’t just rent a hotel/motel room yet, and I was thinking public sex like maybe when it’s really late in the middle of nowhere, I could say I wanna go out on a picnic date and look at the stars and then you know lead on that I wanna fuck from there lol but idkkkkk...