Two things in this post that should never be used in the same sentence, hanging someone and low budget. I would love to see alternatives to using a live actor though, but unless its a prop actor then I would say mods please watch this thread carefully.
If I remember correctly, Bonnie/Erma (depending on the version) just has to appear as if she's upside down looking through a port hole. You wouldn't have to actually suspend her to make this happen. Maybe some sort of bar that she could hook her legs over...
It was 36 years ago but when we did it in HS I believe we just had a second floor riser that the actor leaned over and looked through the portal. I don't really remember the scene but I know we used double floor sets and we flew no one.
You could probably create something with a mirror at a 45 deg angle. She could be on a platform behind the set wall on her belly looking down into a mirror that's behind the porthole.