Convertible Frame Stiffening

All of the unibody rockers create a sort of tubing in shape. The torque boxes on all convertibles and some performance models tie the frame rails to the rockers to in effect create a subframe connector even though it's not in a straight line.
There is still some flex. Adding a straight line subframe connector will stop it.
As far as the aftermarket connectors not fitting a vert.. you may need to do a little notching to fit with the torque boxes.