I am trying to get a small box made to sit on top of tomorrow's cake. I made Wilton's candy clay the other day which has been sitting on my counter wrapped. It is hard as a rock before I start to work with and then softens. After I cut my box sides and stick in fridge, it does harden some but then as it warms up, softens again.

Is there anything I can add to this to keep it hard like a candy bar?

I'm sure this answer is probably too late but it may come in handy if it happens again
I mix a bit of tylose powder into gumpaste. Once it is well distributed, I knead the same amount of the gumpaste with the candy clay. The gumpaste stiffens the clay and it should stay harder, although not as hard as gumpaste alone.

I have never worked with this medium. What proportions are used to say, a 1lb batch or candy clay or fondant? If I am understanding correctly, it is used in conjunction with gumpaste in equal amounts? Does adding it change the flavor in any way?

Gumpaste is actually fondant with tylose powder added to it. It makes the fondant dry hard. Fondant will not dry hard without the tylose.
I add tylose to gumpaste to make it extra hard. So with the softness of the clay and the gumpaste with tylose, your final product should dry hard.
It really doesn't change the flavor of the clay. To me, it justs tastes a bit sweeter.
You mix together equal amounts of gumpaste with clay.
You can also mix equal amounts of gumpaste with fondant. This is called a 50/50 mix.
Personally if I were making a box with stand up sides, I would use straight gumpaste or fondant with tylose added.

You can also make a box by melting the candy melts and spreading them 1/8" thick onto parchment paper. Let that cool to room temperature, then cut. Then put into the fridge to harden completely.

This will remain hard at room temperature like a candy bar. Candy clay doesn't harden up the same in the fridge after you have added the corn syrup.

PS I found the previous post about gumpaste and clay to be very confusing. I wouldn't try adding any gum powder to candy clay except as an experiment with a month before the deadline...

BakingIrene, could you direct me to that post? I'm curious now. I add tylose to fondant/gumpaste and mix it with clay quite often without any problem whatsoever.

Hahaha, now that we have thouroughly confused you. let me try to explain.
I'm sure that Gum powder is another name for tylose. I call it tylose!
I have added tylose to fondant and then mixed it with candy clay,
I have also added tylose to gumpaste and mixed it with candy clay.
This is how I do it and I've never had a problem doing either one.
Tylose does not have to be added to 50/50 because the gumpaste already contains it,but if you wanted to, you could add it.
If your clay is too soft, then you should definitely cut back on the corn syrup.
If I'm using Merkens i use1 lb Merckens Coating Chocolate and 1/3 cup light corn syrup.
For Wilton Candy Melts I use 14 oz melts and 1/3 cup light corn syrup.
I hope I have answered your question of the day!

Okay, that is making some sense. Tylose is already in pre-packaged gumpaste so it doesn't need to be added to 50/50. Right?

Yes JoyceD, that is correct.