What size Stopper should I get?

So I have a vacutech pump 9 x 2.25 inch tube, and I’ve been packing the pump for the last month or so, but it tends to suck up a lot of my scrotum at that point, so I have to stop continuously. I read that a stopper would be perfect for my problem, but I can’t find anywhere that discusses what size I should get? Should the stopper be something that wraps around the penis, or should it be fitted for the inside of the tube?

Vacutech sells some sleeves and stuff.. But I’m not sure that it’s the same thing. They look like clear rings, but I dunno, they look more like cock rings, which I have a lot of bad luck with (always end up too small or too big, I’ve never been able to find the right size). Cock rings also seem to hurt my penis while inside the pump, even though they do a mediocre job of preventing my scrotum from getting sucked in. A stopped looks thick enough to hold more back, but soft enough to conform while pumping (from what I’ve seen at least)

Any suggestions on what I should be looking for?