I was asked to show more detail on the spice bottle box mag modification I did for the G&P Mk23 Stoner 63 box mag. Filling the Stoner box mag was pretty difficult. It required you to slide off a side panel, and then open a small door that was impossible to grip with gloves (very difficult even without gloves!). So to make it easier to fill the box mag, I modified a spice bottle cap to use as a new fill door. I extended the modification to include a full spice bottle. This more than doubled the capacity of the Stoner box mag, and filling was even easier by simply twisting off the bottle cap.
I first cut a hole in the box mag outer box and a matching hole in the inner box.
I used a spice bottle top that had a flip open lid. I attached it to the box mag by using a velcro strap that was riveted to the bottle cap. The cap had to be removable so the box mag could be installed as it slides into the box mag bracket on the Stoner 63.
The extended spice bottle modification was installed the same way, using a velcro strap riveted to the bottom of the bottle. The bottle was painted black. I ended up covering it up with black duct tape, as a BB hit to the bottle cracked it and made a hole (BBs were pouring out everywhere, heh).
Similar modifications can be made to other box mags, you may just have to come up with alternate methods to mount the spice bottle cap. Also by cutting the top of a bottle to include the threads, a twist on cap can be used, instead of the flip up method.