I really like welded steel frames for production molds because they are light(ish) and stiff and easy to handle. The down side is that you have to weld them and they have a different CTE than most composite materials. But the good thing is that you can weld them up before hand off the job if you are working from a CAD model. Square tube works well and is easy to deal with.
Isolating the mold shell from the mold frame is important to prevent hard spots and I usually use low density (5lb or so) foam blocks a few inches thick with a few layers of tabbing over the frame and onto the mold surface. This fastens the shell but allows some thermal isolation and a bit of float. It is good to do this while the mold is still on the plug and demold the supported tool once the frame is tabbed in place.