I just learned about the use of cowry shells as binary dice and find the idea enchanting. However, I'm concerned about the apparent "fairness" of the dice. Obviously, because there are multiple instances, they are not an equivalent to a single die with equal sides, (1d6 vs. 6*d2-1) as they'll have a curved probability distribution, rather than flat probability, and have an incredibly low chance of rolling 0. More than that though, it seems apparent that each shell would have subtle proportional variations depending on the lifestyle of the cowry it came from, and these would likely make each shell prefer one state or another, meaning each instance would be weighted. Also, cowry shells may just, in general, not be especially 50/50 in the first place the way coins are. (I'm OK with that, but the individual weighting bothers me) I can't seem to find any statistical examination of their balance or probability.
If I wanted to use cowry shells as dice in a home made game, what could I do to improve the apparent fairness of their application?