Maybe I have a different answer for you. I didn't use the cotter pin because I like the idea of keeping things simple and I still wanted the ability to take the mags apart, at least as much as you can with a limited one.
So instead of using the cotter pin I just put a notch in the spring retainer so I could move the spring forward a little.
This makes it easy to glue the bottom back on, no spring pressure, and I can easily remove the bottom to pull the spring and follower (on 25 round mags) out for cleaning.

Pics ain't the best, but I'm not a photographer and don't pretend to be.

Here's the retainer (I think that's what its called) with the notch cut into it.



And here it is on the end of the spring waiting to be pushed up into the bottom of the mag.

edit; OOPS! I took the pic and posted this before I put it back together.
That's when I realized that the retainer is actually on the spring backwards.
But it still accurately shows how the spring fits into the new notch.



Doing it this way also eliminates the potential problem of the spring falling into its old position on the retainer and binding up at the bottom on you.

Hope that helps answer your question.