I lost a spool once.....
So I read up and found how other people did it.
Note on the red primary, the boltsnap instead of a double ender. Won't lose that
On the gap reel (made by Marchand on the boards), in the first picture note how even if the double ender swings around, it won't be pushed open. Second shows how if it's put on the opposite way, it will easily open it self and slip off. I know it looks like it could open in the first one, but it won't. In the second picture, with the double ender put the opposite way, the thumb slide thing hits the reel and if pushed the right way (easy to do, especially if you can't see the thing, you can't guard against it) it opens easy.
On the spool, note how the line is pulled through and over. Secondly, note how if the double ender is put so the gate is towards the line, it is much harder for it to be pushed open. I tried to hold it in the second shot to show that if the gate is away from the line, it can be pushed against the spool and open up, hopefully you can see that it's open already in the shot.
So first picture is the right way to do it, where the double ender is less likely to be opened. Second picture shows wrong way and hopefully I got it in a position so you can see how it will be pushed open.