Sorry it has taken so long to get back to this post. Here are some specifics about the curtain pan I have.
It measures 30 1/8" wide by 25 1/2" deep on the top of the pan and where it mounts to the underside of the floor.
The bottom of the box is 27 5/8" wide by 23 1/2" deep (where the door mounts).
This leaves a 1 1/4" flange on each side to mount to the floor with 4 bolts (2 on each side). From the edge of the top of the box (where the door is), the holes are spaced 7/8" and 22 1/2".
There are 7 small nail holes along the edge that the door closes toward. In the pic you can see one of the small holes and the latch hole.
The door measures 27 5/8" x 2 11/16" tall and 1 1/8" deep. It has guides on each side to help shut the door in line with the box. The latch is not set in the center of the door/box.
The hinge is a "piano" style hinge spot welded to the box and door.
Hope this helps. Let me know if you need anything else. I am looking for mounting information (location) of the pan on the floor itself, when I find it will post that as well.