welcome folkwort to AT.
when I first read your post here, I thought to myself, "a portable divination box!" what a great idea! then, I realized, I actually have this myself--well sort of. I carry one with me when I go to read at the New Age Shop.
I do get where you are coming from, and will tell you what I carry in mine. I have this small cloth bag, that is quite sturdy--sort of like an open purse, that a friend bought for me years ago. Inside I keep a tarot deck, sometimes two or three, and some crystals. I also keep a notebook, pen and clipboard in it for my clients to use to take notes. Now, while I understand your divination box is not going to carry the notebook, pen and clipboard most likely--it is how I travel w/my bag. My bag also has various crystals and I also have a rosary inside, and generally a religious medal or two, and perhaps even a prayer card/saint card. I change things around often, depending on how I feel at the time.
I think if I were to have a small box that I could pick up and carry for divination - unrelated to reading at the store- I would include of course, a tarot deck, a scrying crystal, crystals, holy medals, (I am Catholic and carry medals on me all the time), perhaps a reading cloth, (tho I myself do not care much for them), a pendulum is good too and runes. Perhaps a small white candle and matches, and maybe making sure something for each of the four elements- fire, water, air and earth.
I guess since it is your own divination box, anything you feel really drawn to is appropriate. sort of like a traveling altar for yourself.
nice thread! and again, welcome to AT.