Maybe a diaphragm or a cervical cap? They're smaller than a female condom and might not block sensation the same way, but would provide the same safety.
Me, 30ish bi female, been doing solo poly for roughly 5 years. Gia, Clay, and Pike, my partners. Davis, ex/friend/"it's complicated." Eric, Gia's husband. Bee, Gia and Eric's toddler.