I challenged myself to come up with a small journal kit that could fit in an overnight bag or a carry-on piece of luggage with little effort or sacrifice. I find that if I can slide my journal kit into an unused front pocket of my suitcase, then I'm not even going to question whether I "need" to bring it with me.
I started with this inexpensive-but-durable nylon zipper bag . . . it's 12" by 7.5", and it only cost $2.95. I picked this up at Wal-Mart, but I've seen them at office supply stores, too. It's water-resistant, too!
In order to fill the bag with the most crucial stuff for journaling on the go, I thought first about what I use most often in my journal spreads and made a quick list, figuring I could edit the list if I ran out of room. And, you may find that some of my "essentials" don't really matter to you--making your kit a bit smaller, or freeing up a smidge of room for your own must-pack art supply. Once I wrote down my own wants & needs, I then raided my studio and piled up the results:
I use craft-grade acrylic paints instead of gesso on my pages, and via the local Dollar Tree, I found this great six-pot pack of delta paints--cheap and a good way to travel with more than just one color of background paint. One can ALSO find empty paint pots of this sort at any craft/art supply store, so they can be filled with an artist's own preferred colors/brands of paint.
For a wet adhesive, I usually pack a small jar of Inkssentials's matte Glue 'n Seal. This can be used in place of Mod-Podge, and it never dries sticky. I found tiny little samples of JudyKins's Diamond Glaze at a local scrapbooking store, and they are PERFECT for travel, but they're intended to be free samples, so I'm not sure how widely available they are. The small jar of Glue 'n Seal, in contrast, is a widely available size, is approved for airline carry-on travel, and the lid has a built-in application brush (which I love).
In my kit, I also included a small travel pack of hand wipes, a dry-adhesive runner, a small ruler, a few paintbrushes, and a small tin with some broken ends of Caran d'Ache watercolor crayons. And, I put together a small cache of ephemera, images, and vintage book pages for inspiration in a clear plastic envelope (a sandwich bag would work, too). I cannot even explain how hard it was to limit my selections to just this . . . OMG!