Another pump question

So I have this pump that’s a bit too large for now, but it seems to be doing the job until i can get a better one.

One problem I am having though is at the entry hole going into the tube, it seems to hold skin back at the base and when I create vaccum my cock feels very tight and I’m not sure if I’m getting maximum length due to the skin being held back.

Should i go buy a larger donut for the base or try to cut 4 slits into the entry so it can pull more in? I don’t create a mass amount of suction, only 1.5 pulls on the plunger (it doesn’t have a bulb). That creates low tension. but I’m having a problem with the skin because it’s uncomfortable having something pull my cock to it’s maximum but also holding skin back while doing so.

Thanks in advance