It is unknown how many or which GP30s had them or what they are for, but a dynamic brake extended box can be found in front of the dynamic brake fan and is level with the upper roof. They can only be seen from above the unit (kind of like us modelers see models) but they seem to be somewhat common. The first photo is of unformed boxes as they are packaged. This is the top side of boxes. The second photo is of the bottom side of the boxes. Notice the bend lines, some bolt head "spots" and some optional trim lines. The third photo shows a box with the first two bend made. Also notice the bolt heads. These are formed by pressing a dull pin in the "spots" from the bottom.
The fourth photo is a box folded to shape and ready to be installed. The fifth photo shows a box added to a regular Proto2000 GP30 model. Or, photo six, a box added to a Proto2000 GP30 with a GP30 dynamic brake hatch.
The last photo is of a box in front of a Cannon and Co. Dyn. brake fan base sitting on a fan hatch.
#14806 GP30 Dynamic Brake Extender Box (2) - Proto2000 Model