She had kind of expected that. Doesn't make it less unsettling to watch, but at least she wasn't taken by surprise this time. Her next attempt is done on an open box, with the initial "the inside of this box" replaced with "the portion of the inside of this box more than an inch away from the taped-shut side" (all the other sides are either folds in the cardboard, or glue).

This one breaks in the same vision-bewildering way once one side is no longer taped shut.

Palm, meet face. That should be "more than an inch away from the currently taped-shut side". 

That works! Now she has a box that gets to be weirdly cavernous an inch down. The part of it where it swoops out does so neither at a curve nor a right angle but a kind of unappealing compromise between the two.

She'll want to refine that ugly angle before she starts selling these things for top dollar, but other than that this is just what she expected! Nice. She spends the next while flipping the box over, feeling around the inside and outside with both hands, putting things in and taking them out again, examining the ridges in the cardboard to see if they're twice as far apart now, and so forth.

There is some extremely bizarre proprioceptive feedback when she feels around it in certain ways. Things inside behave as though it is cavernous. Cardboard ridges on the inside are twice as far apart; on the outside they are not. The box still weighs as much as all its contents.

Weight reduction is an obvious complement to space expansion, but that will be its own entire series of experiments. She enchants the box for durability without disenchanting it first, and checks that it will still open and shut.

Nope! It is rigid and unmovable and cannot bend the way a cardboard box must.

She makes sure there's nothing in there and takes off both enchantments. How best to avoid this problem depends on the details of the boxes she's designing a workaround for, which in turn requires a trip to the internet. Can she find a source for nice-looking wooden boxes of various sizes with flat sides, hinged lids and somewhere to attach a lock?

She can find a place that sells lacquered jewelry boxes of assorted sizes for really kind of a lot but they're very pretty and they'll ship for no extra charge anywhere in the United States and Canada.

She's already made more than fifteen grand on invisibility jewelry sales. She could buy one of their cheaper ones and call it an investment. One or two more tests first, though. What happens if she fills the box with wads of paper and then enchants it for capaciousness? Does it end up partially-full with the same amount of paper?

Yes. They don't clip through the sides either!

Excellent! That means she can tell whether an expansion enchantment took even when she can't open the box. She disenchants it, makes it durable, then space-folds it (just for depth, rather than bothering with the whole three-dimensional mouthful). Does this work, or just bust the durability?

It doesn't bust the durability and it does work!

Great! One more disenchantment, and this time she cuts off one of the top flaps of the box and puts it back on with a strip of tape positioned like a hinge. Then (after some puzzling over grammar) she hits it with a modified durability incantation: "Make all components of this box durable and strong against damage; let nothing break or alter them except that the tape should still be able to bend." Can she still open and close that flap?

The spell fails completely.

Bothersome! At least she hadn't bought that jewelry box yet. She looks through her French notes again and checks the internet for purchaseable loose hinges.

She can buy them in lots of 144!

Ugh, really? It's not that she can't afford 144 of them, they're just hinges, but her parents might want to know what she wants with them. (Not that they'd open her mail on purpose, but 99% of the mail to this house is for them.) And she'd have to either find something to do with them or throw them away. Forget this, she's going to Lowe's.

Lowe's will sell her hinges in much more reasonable quantities!

Good for Lowe's. However, before she finishes reworking her durability spell into a hopefully-hinge-friendly version, game night comes around again. She shows up looking unhappy.

"What's eating you?" Sanjay asks. (Brenda's late.)

"Well, if anybody else wants to be the wizard this campaign they can have it--I'm moving to Seattle in a month or two."

"- oh, no," says Xavier. "That's super rough. Parent's job moved?"

"No, it's my grandma--she's getting older and she wants my dad nearby instead of across the country. They've been thinking about it for a while and they just got a good deal on a house, and they wanted to jump on it this summer so I don't have to change schools mid-year. I get it, it just kind of stinks."