Plastidip servo's, balloons don't hold up that well when they are exposed. Get the 10" party balloons. Stretch the balloon out before trying to shove something in it. Once you've worked the balloon over a bit, stretch the mouth of it open and drop your receiver in it. Put a little dab of silicone in the neck of the balloon, then put two zipties and create a silicone bubble. That way you know it's air tight.
As for the servo's, just get a can of the plastdip stuff, take off the servo horn, put a screw in the output shaft so it doesn't get all clogged up, then dip the whole servo in the can by dangling it with the servo wire. Take it out, let it drip on a paper plate, but keep it hanging in the air somehow. Wait for it to dry, then dip it again.
Once it's all dry, trim off the excess around the output shaft with a razor blade and you should be good to go.