Well, really, all you need to do is put some RTV on the seams of the servo and around the plug/pigtail wire on the servo. Then find a o-ring that will fit under the horn, then sand the horn a bit so it just gently squeezes the o-ring. It's a good idea to lube the o-ring with associated green slime so it doesn't bunch up.
Or, you can buy plasti-dip and dunk your servo in it. This will take care of the case/pigtail, but you will still need the o-ring under the horn.
Here you can easily see the o-ring:
Once the plasti-dip dries, you just cut around the output shaft and mounting tabs, then peal it off.
I used the dip kind, but I watered it down about 30% with mineral spirits as it was way too thick and bunched up in places on the first one I did. They also make a spray kind.
This method lasted the longest for me, but eventually, the servo still got water in it, I'm guessing through the horn area since that is where the water was most noticed. Then it ran down into the motor.
Or, you can try a balloon, but I find that they tear easily when bashing.