I have the Ranger with a Carter SMB.
On my Ranger I have a couple of unused grommets on the cummerbund (they are adjustment points for the cummerbund). I put two rubber loops sticking out of these grommets. They hold the SMB tight against the cummerbund, parallel to my body. I have the older-style Ranger, with the roll-up pocket, and the SMB is right next to the pocket. To store the SMB I roll it up, pull out the bottom loop, stick one end of the SMB into the loop, then pull the other loop around the other end of the SMB.
I used a ¾ wide rubber strap that I had laying around, but heavy surgical tubing (like you can get in the hardware store) would work just as well.
I ran the two loops through the grommets from the inside of the cummerbund, and, at least with my rubber strap, the knots are large enough to keep them from pulling through when I stretch them to put the SMB in place. If the surgical tubing is small, you might want to try using just one piece, tied in a larger circle. Then tie a big knot in the middle of the circle, so that it forms a figure-8. Put one part of the figure-8 through the top grommet and the other part through the bottom grommet (again, from the inside), so that you end up with the two loops sticking out.
While you are wearing the BC with the cummerbund cinched, you shouldnt have to worry about the loops pulling through, but you still may want to do something on the backside to prevent it if the knots arent big enough.
I dont have my finger spool yet (I plan to only use the SMB as a surface marker until I can get a finger spool rigged and spend some time practicing deploying it). When I get one, my plan is to carry it disconnected from the SMB. Im planning on putting a loop of surgical tubing on the bottom of the snap-hook (where the line is attached) that will stretch over the spool to keep it coiled. Then I plan on attaching the spool to the BC by passing the snap-hook through the bottom SMB loop and clipping it to the top SMB loop. Or I might just stick it in a pocket.
Hope my description makes sense. If you want, I can try to post or pm some pictures.