Google half frame bags and you should have plenty of options. Many that are customized for some bikes so check out if some have been made for your frame.
Since you're an ultralight racer, you must have a bike that wasn't designed especially to bikepack so you could ask some small manufacturers to make a custom one for your bike as well

I am not much of a bikepacker, but have been getting into untra endurance mountain bike racing and brevet style bike rides. I am looking for a 2-3 liter frame pack that keeps as much of the frame open as possible. Apiduara Racing frame pack is the perfect thing for me. I have a couple of Apidura small bags that I love, and don't mind spending the money on good product.

But since this isn't my world, I don't know what else is out there like it. Please help.

Apidura Racing Frame Pack alternative