If choosing to use butter and margarine, keep in mind, that they are solid but very soft at room temperature. Butter and margarine can become rancid if left out for too long in the heat of summer, for example.
Either will work, but if you're making enough mix to store for weeks or even months at a time, you'll need to keep those versions of the baking mix in a very cool place such as your refrigerator.
Personally, my precious refrigerator space is far too limited. The last thing in the world I'll be keeping in my refrigerator is a package of a baking mix for any length of time, let alone months. I will always use a shelf-stable shortening in my premade baking mix.
Now I know someone will say to me, oh, you should only use this type of shortening. Or, tell me, that type of shortening isn't good for you.
I'll take just a moment here to thank social media for helping to create a culture that is afraid of everything. Here is the truth. All shortening is fat, and none of them are all that "good for you." Use what works best for your situation and enjoy life.
You can use a large mixing bowl and a handheld whisk or a pastry cutter to cut in the shortening when making the mix. My personal preference is just using the whisk.
If you're making a baking mix in bulk, say a few pounds of flour at a time, you may also choose to use a stand mixer to cut in your shortening as well.
Homemade mixes of any kind do not come with a box. You can use storage bags to store your mix, however, just like bags of flour, they are prone to ripping open. You're better off finding a rigid, airtight container that will stand the test of time.
Another consideration is recipes. When you buy a box of mix from the store they often come with several basic recipes conveniently printed right on the box. If you like having recipes right on the package, try printing them on large address labels and sticking them on your package.