Step 2: Push Down and Turn - 2
Don't put the thumbtack in the center. Put it close to the edge, but not right on the edge. If it's right on the edge, and slips between the caps, it sometimes won't catch the inner cap.
If the outer cap is really hard plastic it can sometimes be difficult to get the thumbtack through. You could just tap it in with a hammer or some other hard object, like that can of lima beans from 1978. On the hard ones, I've found just twisting the thumbtack a little, while putting pressure on it, will get it through pretty easily.
You could just put the thumbtack on the table with the point up. Turn the bottle over and push it straight down onto the tack. If the pressure isn't even, it's easy to bend the thumbtack. To keep the pressure even, just use three thumbtacks. Space them evenly around the edge and push the bottle down on them. Even if only one goes in, that's enough to lock the caps.
The only expense is a thumbtack, and what are those worth? Less than a penny? And it takes all of 15 seconds.