In order to clamp the endpieces together for the A123 20Ah, you have two choices.
One, buy an expensive clamping tool which will apply even and heavy pressure to the pouch cells to keep the chemistry evenly distributed and working effectively. Something like the Band-It tool
Another option is to use hose clamps. Hose clamps aren't nearly as strong as the Band-it tool, however, there are a couple of advantages. First, no expensive tools needed. Second, they are removable. The Band-It tool's clamps need to be cut off and cutting a band that is under great pressure isn't very safe. Third, they can be bought fairly cheaply.
I found the MakeAClamp
system online. It's a very flexible system in that it allows you to specify how large you want the clamp to be. The system is based on clamps that can be re-positioned and bands that have notches cut in them. I bought one kit which contains 3 clamps and 10 feet of banding material.