I use various kinds of boxes that my husband has accumulated over the years. One is a large rectangular margarine tub, one a part of a multi-piece lunch kit, and the others are like the ones below. They are a mixed bag of different things. Believe me, it's nothing fancy. But really, any kind of plastic box that is a good transportable size and is tall enough for flavor bottles will work.
There is room in the softside cube to stack up at least six of these boxes or more, and still have some room for extra PG/VG or empty bottles.
Several of my boxes are similar to these:
The lunch kit box is like the one on the bottom in this picture:
Softside cube is similar to this one, I think these are something like 14" square:
Wire basket like this, about a foot long, I stack all the boxes up and then just sit the basket on top:
My wire basket has become cluttered over time - it contains flavors I don't have room for in the other boxes, plus working-size bottles of nic, PG, and VG, and my scale. Most of my TFA 30ml bottles are in this basket because I don't have room for them in the box they should be in.
Every now and then as flavor collection grows I have to get another box or two and rearrange things. Right now I need to divide TFA flavors into two groups, am still thinking about what groupings make sense and how I can get about an equal number of flavor bottles in the two groups.
Maybe one of these days I'll spring for a collection of boxes that match!