Hi Boston. I had no problem with the battery being loose. I'm using the Efest 2000mah button tops with the smaller end cap. Yes, i did receive 2 end caps. Larger one for stacking the 18350's, and the small one for 18650's. I would contact Abe, I'm sure he will correct your issue and send you the other end cap. I've contacted him a few times, and he was very good at getting back to me. Sounds like you got the larger end cap, since you had to stretch it out. Which battery are you using ?
PS. Just tried the larger cap out of curiosity, and works fine with the 18650, no rattles. Are you using flat top
batteries ? Maybe the reason. Idk. Had another member on here, warn me about too long of a battery also. He said, if the battery is too long, it can break loose the contact up in the head unit, said it's just a small tac of solder that holds it.