If you go search regularly, there are some cheap two-dot-two you’ll find that are ascii. Bit longer in string length but shorter on the wire.

I own 0e.vc, which is on GitHub as a general purpose xss domain if you need it. Iirc it does eval(window.name), or location.hash. Whatever works for you. It’s also on the public suffix list which makes it almost like a top level domain for security purposes. So I can have subdomains that can’t ever share cookies :-))