EDIT; Feedback from guests is that they love that I offer a King bed.
I have two twin beds that are set up as a King in my listing. I have “zipped” them together using a mattress pad and the foam T insert to fill in the gap, then a 3" memory foam topper. So far all guests except one have loved the mattress. I did have someone want to stay and use the twin configuration at my regular price, but they canceled.
I think I might make a separate listing with photos of the twin setup and maybe charge $5 more? Or a bit more on the cleaning fee?