Hi @king of nowhere @theJesuit. Thank you for your replies.
I did not notice that I am looking for an orbit for the Kerbin orbital station, where the inner radiation belt is more complex and has a sickle-shaped cross-section. With a periapsis of 72 km, apoapsis of 625 km and inclination of 49°, I am already crossing the horns of the inner belt, and still cross the outer belt. Any change moves my orbit into one of the belts. I can try diving into the atmosphere, but do not want to use that kind of cheating.