Hi ydelle,
If you put slats on a flexible box spring then they would still be subject to the flex of the box spring itself and they may also be subject to shifting and moving under the mattress. The box spring by itself would probably have better ventilation than the box spring with slats on top of it. If your box spring is still in near new condition and has no dips or soft spots (you can test this with your knees or by pushing down very firmly and evenly in every area of the box spring) then it would probably be OK to use (as long as it met the warranty requirements of your mattress manufacturer) but an active box spring may also change how your mattress feels and performs compared to a non flexing foundation.
A plywood sheet would provide a more solid non flexing support system (which may still be subject to some sway) but of course it would also lower the ventilation of the mattress between the plywood and the mattress. You can see some of my thoughts about a solid surface under an all latex mattress in post #10 here.
If you mean a steel frame to support your boxspring then it would depend on the frame. Some of them can be adjusted to different sizes and some can’t.
Phoenix