After consulting one of our C2C Care Experts, I have some suggestions.
The preferred option for preservation would be to take the doll out of the box. First, take some documentary photographs to preserve what it looked like in its original format.
You’re correct that the doll should be stored flat in a box that passes the PAT test. Within the new box, use stuffed knit tubing to secure the doll from movement. You can use a lightweight Hollytex (spunbound polyester fabric) to pad under the dress and any other areas to relieve pressure on the fabric.
If you think it is crucial to keep the doll in the original box our forum expert recommends using acid-free 20 point folder stock cut to the size of the acidic box’s side panels to act as a barrier between the box and the doll. Then use light weight Hollytex fabric to loosely pack around the doll and inside the box. You could very carefully pass the Hollytex behind or underneath the doll if possible. The same fabric can be used to lightly stuff the dress as well. You can place the entire box inside an acid-free box for long term flat storage.
Please let us know if you have any other questions.