Don't. Flying a Klingon flag is a recipe disaster. Pretty much any non Trekkie is going to think you are a nazi

Depending on if you could have a print shop make one for you, maybe a white flag with the Klingon-cardasian alliance emblem instead?

http://images1.wikia.nocookie.net/...

Along the same vein, here's all the cannon emblems according to the wiki

Hell if I know how expensive a custom flag is to get printed but or both look pretty good. Worry less about canonicity and more about if the aesthetics work for you.

https://www.maskworld.com/...

Maybe?

So what I think you want is the DS9 era Klingon emblem.

Check out this eBay listing: https://www.ebay.com/...

According to this article this is the banner that was used on the Promenade in DS9. This is a “chair drape” (whatever that is) but maybe you could use it as a flag, convert it to a flag, or have a custom flag made based on this?

not the very, very nazi looking red-white-bla

Fascinating, I never would have thought a Klingon flag was Nazi.