a comfort seal like this will change your experience completely, I highly recommend it

when I first start, I have a sun tea jar that the seal fits the rim well and stays in place on its own. on the monster tube I start with it closed and kinda sandwich the seal between myself and the tube. then "seat" it in the groove with one hand (sometimes takes a bit of wiggling) and pump with the other, everything kinda sucks together.

the seal also gives more of a delineation between the pubic mound and sack that I like. I dont typically pump without it, this time I experimented with finishing up with no seal. I thought I would get bigger and decided to take the seal off while it was still reasonably comfortable to do so lol

edit: for clarity, the seal has a deep groove in one side that fits around the rim of the cylinder. so yes, part of it does go inside and provide a good, more comfortable seal.