I’ve been very pleased with white Plasti-dip.
I think it is better than black, seems to hold up better. You can get a can of the liquid (not spray) for about $7.00 usd at Home Depot or Harbor Freight. It will last forever!
What is great about this is you can dip as many coats as you need to narrow down the mouth of the tube, it not only makes for a much better, easier seal, it can also act as a cockring and increase the internal pressure of your penis when pumping, allowing better results with less vacuum.
If you no longer want it, or if you want to redo it, it is a simple matter to peel it off the tube.
I have used it on my gravel filter tube, and it has really made it a great cylinder. My only regret is that I didn’t try this 2 years ago!
With this, I really think you could buy an oversize tube and just dip it to make the entry perfect, then just remove it or redo it as you get bigger.
When you go to put on multiple layers, you only have to wait 20-30 minutes between them. When you go to use it, make sure you let it cure overnite to be sure.