Just a suggestion, but approach it with a bit of care. Wrap the base of the glans just a bit tighter, and get a bit of an erection to push the blood into the glans to expand it. Once the glans expands a bit, start the vacuuming. Be very careful to use low pressure first, since the glans is already at a hyper expansion state. This will hold the pressure much more tightly. The expanded glans also acts as a mushroom anchor, similar to those who use it for kayaking and boating. In the water, the inverted mushroom head acts as a friction system to generate a downward force, or, in a vacuum system, an anti-retraction force in the cup.