That's very unncommon and sounds awkward to some, better not to use it. The adjective should agree with noun in its grammatical case..
You may say it, you might hear Poles say it, it is not correct, unless before a noun.
How do you make the right letter l? My keyboard is in American/english/normal
You change the keyboard to a Polish one, as it has everything you need for English + of course everything you need for Polish ;) And then it's "right alt + l" or "left ctrl + left alt + l".
Do adjectives match with their nouns in their grammatcal case even after verb "być"?