how to make cake denser

That is a great question!  There are several ways to modify a boxed cake mix to make it more dense. You can try any of the following: adding a 1 oz. package of pudding (any flavor that complements the cake’s flavor profile), adding 1-2 egg yolks (along with the eggs called for on the box), or substituting milk for the water called for on the box.  I hope this is helpful to you.  Happy baking!