A 5 pack of High Temperature Silicone O-rings to hold your Solo mouthpiece firmly in place. Now you can have a tight fitting stem every time that won't fall out.
Just remove the top screw on cap from the Solo, center the O-ring over the heater port and screw the Solo top cap back on while keeping the O-ring centered. Do not overtighten.
WARNING Do not install the O-ring on a hot Solo, let it cool completely to avoid burning yourself
Some Solo models with a thicker cap will not be able to thread the cap on properly.