Attaching Shaft to A1 Servo Spline

Hi, I’ve never worked with servos before. I was looking at purchasing some tiny servos that have an A1 spline size. When I look for gears or couplers to attach to it, I only C1 size. Are there A1-sized connectors that will fit on the spline anywhere? This is for an RC submarine. I want to use the servo to manipulate dive planes. It needs a rod to pierce the hull, and that point needs to be water tight.

Unfortunately, we only have a C1 size coupler. You could try 3D printing a coupler to try and fit this size.