If you know what you're looking at, you already know what the problem is, and how the improvised clip solves it. Nonetheless, let's take a closer look.
Step 1: EEEW! THAT'S GOONA MAKE a MESS!
This standard two-part epoxy syringe has a maximally irritating design flaw. There's no snap-like mechanism to retain the cap. Why anyone qualified to design this fairly sophisticated device would have thought that people would be fine with just setting the cap lightly on the end of the tubes, placing the tubes on a dedicated shelf in their cabinet, and walking away defies reason. I'm not prepared to accept that they were too stupid to realize that this is the only way to avoid making a huge mess with this device, or too evil to care. Yet, as far as I know, this packaging is still in production, and has been since around the time the first Terminator movie came out, without anybody rectifying the problem. I guess it's up to us.
Step 2: Just Go Ahead and Fix It.
Not much to explain here, take a used bicycle spoke (any bike shop will have buckets of them, and I'd be surprised if they refused to give you a few just for stopping by) and bend it into the shape seen here. The acute angles will give the clip some spring preload. Match the longest edge to the distance from the finger thingies to the center thingy of the cap.... I don't know the names of the specific parts of the tube. If you're having trouble, just contact me and I'll do it for you.
And voila! You now have one less thing to worry about exploding in your toolbox!