I have done a little more work on the frames. The steel frames were bowing like a banana from being milled on one side and that was using hot rolled steel.
I went back to the aluminum frame with a screw in weight trays or steel weight, haven't decided yet. I have added a couple of holes at the ends of the frame for posts like the one in the assembly rendering. The post can hold a tray that can support batteries or a decoder.
The rendering shows a weight tray on the left, and a steel weight on the right. I will choose one or the other and include them with the frame.
I will keep you posted.