Thanks for the reply!
I wound up using some cable grips to attach the 3/8" rubber tube but I do want to revisit this design as it worked so well for the smaller rope. There was some variation in size it could accept as is and it works great for shoelaces. I should make a bunch of these to tighten tent fly lines for camping (tents and tArps and raising bear bags and such things).
I think it could possibly be scaled in one direction only (possibly selectively scaled) to keep the footprint the same size. But I wasn't sure what things would be thrown off by doing that so I haven't braved it -- yet.