Initially I figured I might be able to cap the ends of the frame with metal by welding or bending some excess material. After talking to a few people familiar with metalworking however, I found neither of these were particularly good options: welding would destroy the zinc coating on the metal, and bending would be hard to do precisely with the tools we had available on an object this size and shape.

3D printing to the rescue.

End caps

After a few corrections with digital callipers to match the real world object, it was quick to make up some parts that were exactly the right size:

The outside of these caps is sanded with 180, then 360 grit to give a smooth matte finish (I'm a big fan of sanding 3D printed parts for this effect). I might get some finer grit sandpaper to polish out the scratches later.

Rear access port covers

The first rear cover I printed was fractionally too small and didn't friction fit like it needs to. I haven't printed this again since revising. The revised version (image) is also slightly thinner, which should help reduce the slight warping I'm seeing from printing these in ABS.

Light sensor cover

The light sensor hole is covered with a small, friction fit plug printed in semi-transparent plastic. The plastic I used doesn't have great light transmission, but this can be corrected for in software, or with a change of dividing resistor later.

Test PCB mounts

The PCB(s) for the segment displays will need to be held in place rigidly and be well aligned. I've done a quick test to confirm the mounting solution I had in mind would work. Test successful assuming a 40mm tall PCB. Friction fit seems to be enough:

Centering a 1.6mm thick board in the frame leaves 9mm clearance in the back, and 1.5mm in the front with the segments mounted. The front clearance should be just enough to fit coloured filters flush against the digits to increase contrast. The back clearance will hopefully be enough to fit a separate control board and any other components or power circuitry needed.