That could work, would depend on the torque the plastic piece is able to transfer, I would also be a little concerned about concetricity. Ideally the shaft is “Independent”(jaw coupling ir similar) of the gearbox, but ideally that requires a dobble ballbearing that keeps the prop shaft from wabbeling. Using a plastic joint should be an ok compromise where some rigidityis left that account for the misalignment
Our design with a rigid shaft coupler with only one ball bearing is a hack. It works (if put together very carfully), but not ideal. As a shaft coupler will never be 100% straight, there will always be some misalignment, even tho it’s not visible for the eye.