I just received my Oskar's treasure chest + jewel from Meffert's. It's a pretty cool concept... though I found a flaw: you don't have to solve the entire puzzle to open the box, just the top two layers. Is this intentional, or was it an oversight? I proved this by doing a 3-corner cycle on the bottom layer, and it still opened no problem. Same with a 3-edge cycle. Upon closer inspection, it's obvious that locking mechanism is only keyed to the top two layers.

Here's a challenge for you geniuses out there: can you modify/extend the existing mechanism so that it does depend on all 3 layers?

This is well known. You can simply impose your own rule that it has to be solved.

I think that one of the solutions to make all 3 layers work in the locking mechanism is to make the 4 middle-layer edges parts of the cover.

You actually don't even have to solve the two top layers... try doing: RL' D2 R'L

This may be getting off topic a bit, but I may have thought of a solution. As far as I know there a notches in the center layer that align with the holes in the top layer. Could you change the shape/size of the pieces that stick out so, say the wrg corner only opens on the rg edge. I hope this made sense, it’s rather late.