A simple and effective way to prevent your balls from being pulled into the tube is use of a leather strap, silicone rubber or metal ring. The leather strap is easiest, safer, and more adjustable than the ring.
You need to apply the strap tightly enough to prevent a testicle from popping up above the strap.
Single leather straps come in three, five and ten snap versions, as well as buckle and velcro closure versions. I prefer the five snap version, as it's most versitle.