I did this with just a Tinkercad, Illustrator, a ruler and a camera because the batteries have been dead for months in my digital calipers, and because my CAD skills aren't that stellar.
I decided that it'd be easiest to trace the shape in 2D and extract the 3D model from that. Tinkercad lets you import SVGs, so making one was my first goal.
To make the SVG, I first took a photo of the shape of the triangle-shaped hole through which the missing piece fits. I tried to line this up as squarely and directly below the camera lens to avoid getting any perspective distortion. There's a bit of slack between the piece and the space it fits through, so perfect sizing wasn't absolutely crucial.
I then opened it in Illustrator, and using the Pen tool, created anchors on either side of the points of the triangle, then using the handles, replicated the curves of the sides and corners. I'm not an Illustrator Pro, so there might be a more effective way of doing this, but it was pretty easy. I also made a copy on a second layer, rotated it 120º and put it directly over the first tracing to try to average out any distortion and get a fairly consistent size/shape.
Next, I imported this SVG into Tinkercad (tip: make sure it's black on white) and resized it to approximately 2" on the side, as hand-measured on the box with a ruler. I had to rotate this to get the side of the shape square to make the ruler tool accurate, and also had to convert 2" to millimeters. I also added a center hole to match the other shapes.
Once in, I gave this about 5mm of thickness, then printed it to see if the general shape/size would fit.
It did, so I went back into Tinkercad and reformatted the shape to make it 37mm tall, then created copies of the triangle, sized it down slightly, and placed it in the center (top and bottom) of the shape to carve out the gaps but leave the center membrane. I eyeballed the dimensions, making the walls about 3mm thick.
Exported this shape as an STL and sent to my printer. Took about an hour to print. I did one face-up, with support material underneath, and another on its side, with no support material — each alignment has its pros/cons, but both work.
Took the piece to the cover, lined it up, and dropped it through — success! We now have replacements for the missing triangles, and everyone's happy — wife, son, and myself.