What tips are you using with the ZST? Have you tried Starline tips, or going 1 size larger to prevent them from falling out? The ZST body is a lot smaller compared to the ZSR, so I'm a little surprised that you're having fit issues with the ZST. The ZSA is pretty compact; perhaps that would work for you? Also, the EDR1 is a cylindrical piston style earphone, and I know that would fit well. It sounds amazing too, and is only about $4usd!