stick w/ 3/4" all the way. Why 3/4" external and 1/2" for ports? wtf? each and every piece of the box will be as important as the other.
speaking if MDF. Stupid mother****ers @ home depot cannot cut for shit. 2nd ****ign time they messed up a cut. Even just by 1/4". I even told ******* to double check. it's a **** shame to have a marvelous saw like that and have messed up cuts!
to stay on topic: 3/4 will provide you with more durability than 1/2 and also less flex. 1/4" is a big difference in mass when it comes to MDF.