Did you remove the factory fit spacer ring ?
Not clear from your original post but when you remove the original driver, there's a small plastic reducer/spacer which seals the front gasket to the door card. Has this been removed ?
On the string idea... wouldn't use it myself. It won't last as long as a ply or MDF baffle. Seems a shame to spend all that money on some high quality drivers then compromise on the installation - biggest cause of dissatisfaction imo for people that have ivested sometimes huge amounts in their product is through poor install or setup.
Also, around the factory fit driver, there are a lot of gaps where the fabric and plastic trims are inter and overlapped. Sealing these with some Dynamat or even some polyester gel coat helps improve the mid bass tremendously. You could go the full hog and glass the new baffles in thereby sealing them fully and at the same time contimuing it across to strengthen the panel a bit more....
The door pockets resonate where the liner is sperate from the actual trimmed outer skin...but you need to dampen the rest of the door a lot to get to that level of pickiness.