I took the original creation,located here https://www.thingiverse.com/thing:2909541 and used Tinkercad to add a 5.5mm gap between the internal bearing and external wall. This allows you to add one 6mm bb at a time. After the 4th bb it is quite difficult to press them in, use a vice or clamp to slowly press the bb's through the hole.
Sand both parts for smoother operation.
V2 made access hole to 5.5mm for easier bb placement