I am assuming you are discussing monkeybar’s device. I improved it somewhat just yesterday and may suggest it to monkeybar:
Closer to the top of the clear plastic, domed cap, I drilled through and added an air passing nipple to which I attach my pumpers (Auto brake bleeding), pump tube. I put a small tab of thicker cloth in the dome to distribute the vacuum over the majority of the internal surface. This surface will be totallyconformed to by your glans after raising the vacuum inside. So after your silicone sleeve is in place and your dick is in place, pump to about 15 hg and you can see your glans inside pressed to the entire surface. O course, your glans may have a larger volume than the dome, so, the dome shape should best be slightly smaller but the same general shape as your dick’s glans shape.
The result after trying this out was marvelous, ( I do not hang—just getting my equipment in shape so when my “all hands” routine fades and plateaus).. I hung for 10 minute @4 lbs. There was slight swelling but the glans shape was better than my normal shape and I believe that if the dome shape was more customized to me, the results would be marvelous.