This canning jar box is going to be solid wood throughout. It doesn’t have to be. You can pretty easily put a plywood bottom in rather than a solid panel, and dividers is one of the very few things I’ve found hardboard any good for. Again, this is a case of being moderately correct.
Mason jars come in several sizes, and most of them will require a differently sized box. It’s a fairly simple exercise to figure that out for yourself. Measure the diameter of the largest bit of the jar, then multiply by the number of jars (3 jars wide, 4 jars long). Add in 1/4″ per section of divider (or 1/2″ in width and 3/4″ in length). Add about 3/16″ for fudge factor. This is your inside dimension. Add the thickness of your stock on both sides, and that’s your external dimension. AND THEN, before you do anything else, write everything down so you don’t have to do any more measuring! Invariably you’ll need more than one box, and you don’t want to have to do this more than once. For most of my boxes, I use 3/8″ thick stock sides and 7/16″ thick bottoms. Quart boxes need to have thicker stock (1/2″ thick sides and 9/16″ thick bottoms) since the jars are heavier. Dividers remain at 1/4″ thick, no matter the jar size (and are Douglas Fir in this case). For this box, which is for wide mouth half-pints, my sides are 15 1/2″ long and the ends are 11 3/4″. Both are 3 1/2″ wide, and made from yellow pine.
Once I have everything planed down to size, then I start with dovetails. IT IS IMPORTANT that you make the half-pins on the ends 1/2″ wide. It would need to be 5/8″ on a quart box.
And before you shut off the shop lights for the night, be sure that you’ve got the bottom panel glued and clamped up so it will be ready to fit to the rest of the box next time.