The Shorty is 4″. I cut it down per Wayne’s instruction shortly after I got the Venus – 4 or 5 years ago. I can’t remember why now. I think I rolled the receiver to get a felt pen line squarely on it and used a hack saw, very carefully. I get most of me into it, I’m kinda small. Haven’t noticed any bumping against the end. Because of the reduced air space it has a noticeably livelier action. I don’t use it every time.
I would try cutting a receiver down to 5″ first off. Remember to sand the edges carefully.
I tried Wayne’s idea of O rings in the constrictors. That would be similar to using the Handsome gizmos as constrictors as you suggest, I’ve always used the Handsome’s on the outside at the mouth. I’ll have to give your idea a try. O rings don’t have a lot of ‘give’ to them, these might work better. They certainly do the job mounted on the outside!