I designed a frame brace for the Ender 3 Pro. The brace connects the center frame extrusion to one of the outer extrusions. I printed three. I installed two on the rear of my printer (one left and one right) and the third unit on the front, opposite the electronics box. If your electronics are relocated you could conceivably install four of these braces. 

Install them as far from the center of the printer as possible for maximum bracing effect. For maximum strength I suggest that you increase the number of top/bottom layers and the number of walls. You can also increase the infill percentage, but more walls helps more. 

If you use one near the heat bed wires you may need to play with the placement a little bit. I spun up a revised model that has a hole in it for the heat bed wiring to pass through. I also made another variation that has a little groove in the bottom to route the ribbon cable for the Ender 3 Pro display. 

I haven't done any input shaping work on my Ender 3 Pro, but I would love to hear if adding these braces has any effect on the results. 

You will need some six M5 machine screws (8-10mm) and six M5 t-nuts per brace.  These fit pretty tight. My advice is to remove the end caps on the 4040 extrusions, loosely install the screws and the t-nuts onto the braces, and carefully slide the t-nuts into the grooves on the extrusions.