Some things are so much fun to make that it's sad when they go together properly first time as I'd like to do them again. Such it is that the radar reflector on the Brede is attempt number 1 but I might do another one anyway.
"Corner reflectors" work by reflecting the radar signal back at the source but "shifted" because the signal has bounced off three surfaces - or by magic/witchcraft if you are a Daily Mail reader.
The real devices are fitted to boats too small to generate a good radar signature so other vessels and the coastguard can spot them. Should you feel the need, you can can buy one from Amazon. Probably not a good thing to dangle from the rear view mirror on your stealth fighter.
Anyway, my version is made from 3 plasticard disks cut by running a pair of dividers around on the sheet surface and then finishing off with a sharp knife. Slots are cut into two of the disks and they slide together. The final disk is split into quarters and fixed in place with liquid glue. Overall diameter for the Brede is 20mm.