Commission question (Patreon)

Here's a commission I'm working on. The big girl is Dawn, a huuuuuge OC of the commisioner. The other girl is normal sized and designed by me. Now, I would like to change Dawn into someone else so that I can use it here, I'm just not sure into who... I don't want to redraw a ton and add a big outfit etc... I wanna keep her mostly nude. The woman getting railed is an anthropologist, so would have to keep this theme to some extent...