So this wedding I'm going to is formal

If you're ok to stand out then yeah I wouldn't care. You're just in the audience.

You can even use it in your wedding speech. "I always mess with the groom and today is no different, therefor my very none formal outfit to fit the rest of my speech which will be a series of burns so be ready, this will get hot"

Try the formal wear hire places.
The price will probably be affordable.

If a tux is requested that's what you should wear.