You are correct, in Swedish the plural form for kubb is kubbar. I'm not shure if its a bad thing to use english endings, its probably for the better.
Otherwise, here goes:
Kubb = Kubb
The kubb = kubben
Kubbs = kubbar
The kubbs = kubbarna
The kubbs color, singular = kubbens
The kubbs color, plural = kubbarnas
I suggest you go for kubb and kubbs in english!