If one really hated his clan tartan, but wanted to wear a clan tartan, he could check for weathered, ancient, muted, dress or other variants. Sadly, not an option for Birse, but Birse is a very nice tartan.

I have the inverse problem. I am a Stewart, so many of my family tartans are pretty-much considered universal. I have my choice of a couple of dozen public domain Stewart tartans, counting the old sett and fashion tartans in ancient, weathered, muted, reproduction and modern colours. Many of them are regularly stocked items. On top of that, there are tartans for Stewarts from several specific Scottish regions. The nice part is that I can easily rent Stewart hunting for formal occasions while I save for my own kit.

A Stewart is unlikely to assume that because you are wearing a Stewart tartan that you are a Stewart. We see "our" tartans everywhere, including on cookie tins.

(If your real question is "What should I do since I can't afford my clan tartan?" I think Black Watch is even closer to Birse than Stewart hunting.)