I have a Size Doctor, which works similarly to the PMP. I'm using it with an elastic strap and leg band, mostly as an "anti-turtling device."
I didn't need or want much tension, just to keep my shaft extended. But even with the lightest tension, the vacuum cap would sloooowly slip off. But with more tension, it stayed in place.
My theory is that the amount of vacuum in the cap is dependent on A) initial vacuum from the bulb and
you get *more* vacuum as you try to pull it off the shaft, since the air space in the cap is very small.
So, long way around, more tension on the extender might help it stay in place.