This is an adapter for this paint holder to accommodate Army Painter paints: Multiboard: Multipoint Paint Holder - Army Painter, P3, Vallejo, ProAcryl

While it can technically fit these paints without the cradle, it's designed to accommodate ProAcryl bottles which are much larger, so Army Painter bottles fit very loosely. This piece simply drops into a slot to make the Army Painter bottles fit better.

From my tests, the fit is just tight enough to stay put when messing with the bottles but just loose enough that you can remove it if needed. You could glue it in, but I don't think it's necessary.

This hasn't been tested with Vallejo paints. I wouldn't expect it to fit since I think those are ever so slightly bigger, but if someone tries it feel free to let me know. I can theoretically modify the model to made space for them, I just don't have any Vallejo paint to test this with.