How can I fabricate a custom one-off plastic part?
That piece looks to just be a single curve (bent in a single axis), so it's possible to do yourself, but it's not necessarily the easiest thing to do, and you can get some optical distortion depending on how evenly you flex it.

If I were to do it, I'd do the following:


Trace the inside contour onto a piece of wood.
Make multiple wood pieces, slightly under size, one for each 6" or so of the length.
Attach them all with some sort of stringer, or just mount down to a piece of plywood.
Stretch a piece of thin metal (look for aluminum flashing at the hardware store) over the plug.
Make sure the existing piece fits on the plug well, if not, adjust.
Secure the aluminum flashing.  (I recommend contact cement or other adhesive so you don't leave dimples from nails or screws)
Get a piece of plastic the length you need, and maybe a tad wider than you think you need.
Get a few strips of wood the length needed, and some clamps.
Place one side of the plastic against the plug, and clamp in place (use the wooden strip, so you're not clamping against the plastic).
Using a heat gun, evenly heat the plastic at the curve, and slowly apply pressure on the unclamped side.
Once you get it bent, clamp the other side and let cool.


If you have a more complex shape, you can try to find someone who has a vaccuum table (or make your own), or find someone with a large enough autoclave.

And one other thing to note -- flourescent lights give off more UV than incandescent bulbs, so most shrouds for them are UV blocking.