The easiest way is to use a plastic tip, 3/8" size. Available at ACE hardware as part # 52504. Another way is to remove the needle from one of your regular needle holders and use that as a cap. You may have to also cut off the long tip and seal it with Duco cement, depending on the exact length of your holder. Finally, you can cut the pointy end off of the regular Byetta cap, and then glue in one of the ACE plastic tips.