Yes, that's the rack and it's "toplight flat plus".

I took a piece of ABS plastic sheet, about 1/8-1/4 inch thick. I imagine acrylic or other kinds of plastic would do. You want something that's fairly stiff but not brittle and easy to saw and drill. I'm not sure where you can buy ABS sheets. I had some laying around which came with another project.

It's easy enough to make a template out of cardboard. I could get the dimensions for you but basically you're just making a square for three holes.

One hole is for hanging the plate from the rack. It's attached using a few washers and a screw. The plate is hanging behind the rack so that the light isn't too exposed and hopefully won't get whacked when the bike is folded. Use threadlock to keep the screw from backing out.

Two holes are for the light. Use the default spacing. 50mm? Also, you want to have the light far enough down so that it can stick out around the outside of the rack, which is narrower than the light. I think the light comes with locknut nuts and washers.

What you want to watch out for is cracking and fatiguing around the screw holes. I haven't seen any trouble with the installation so far.