Hi Pheonix,
We are in Houston and stopped by a local builder. They had this memory foam mattress in the show room that we liked. I mentioned the heat concerns, and he suggested replacing top layer with latex.
If you can clarify the type of foam in each of the layers (memory foam or gel memory foam, latex, or polyfoam) and the type and blend of the latex layer and any other missing layers or components (I’m guessing the mattress is more than 7" thick) I’d be happy to make some comments.