For Germany, there actually once was a suggestion for a flag in that design. Maybe you could use this one?
https://en.wikipedia.org/wiki/Wirmer_Flag
(Although it might be a bit tricky as nowadays, it is sometimes used by some extreme right activists - which is ironic, because it was invented by a nazi opponent.)
https://en.wikipedia.org/wiki/Wirmer_Flag
(Although it might be a bit tricky as nowadays, it is sometimes used by some extreme right activists - which is ironic, because it was invented by a nazi opponent.)