As an experiment, manually stretch straight out, then put on an
extender base and manually stretch straight out. For me, I can stretch further without the base, and it's not that I'm really thick, but my ligaments are fully stretched out, and the
extender base keeps me from fully stretching. I've only seen 2 large
extender bases: x4's girth base, and the Static Stretcher Heavy Duty. The SSHD is actually too wide, and it makes it difficult to walk with, so the x4 girth base is the best option, although I did have to make a modification to it to increase comfort. The PM Pro fits on the x4, and when I get my new glans chamber I'll be using the girth base with it.
Also, the general consensus with
extenders is to not use max tension and go for as much time as possible, but I had success with the FastSize doing the opposite. For 2 years I would only use the noose-style
extender for 1-2 sets a week (not a typo) with each set being an hour or as long as I could go at max tension. I gained an inch during that time, and it may have helped that I was only 19 when I started. I also never did any manual stretching or jelqing until after those 2 years.