I use a latex balloon 'strip' to adjust the tightness of the 0-ring seal in the e-tube. Here is what I mean:
The balloon I chose is pretty big. It's the 'clear' one and measures 7" long (the white one is a 'normal' balloon. 5" long. It is just for comparison)
I sliced up the balloon in wide(ish) strips. One end of my tube fits tighter than the other end, so for it I used a strip from closer to the opening. It stretches more and fits the tighter end better. Obviously I used a strip from the middle for the loose end.
I put the strip around the end cap like this.
I found it easier to place if the strip is a bit wider and I leave the excess around the edge.
I don't have a picture (because I just thought of this now) but use a q-tip to add a little grease before placing the strip. Don't put much on though because it is CRAZY HARD to place the strip when it is covered in grease. :D
Now place grease liberally around the o-ring as usual.
Ding! Nice tight seal.
If it isn't tight enough, use a strip with a larger circumference. You can also use more than one strip if it is still not tight enough. I doubled them up when I was first trying this, before it occurred to me to use a larger strip. Doubling works just as well, so its a good option if you can only get smaller balloons.
I think the latex will eventually be broken down by grease, but for me the teflon tape broke down quite quickly and needed to be redone. I think this will last longer, but only time will tell!