You can use MgCl2.4H2O instead of MgCl2.6H2O. If you prepare a certainty molarity of MgCl2 solution, use the anhydrous MgCl2 is much better that hydrates of MgCl2 because the hydrates are difficult to weigh for their strong water abosorption.
You may want to check my post elsewhere at this forum; addressing the effect of air humidity on magnesium chloride (MgCl2) or its hydrates: https://www.researchgate.net/post/weight_equivalence_chemistry