In general any box works, just find the pallette, water cup, brush holder which work for you. Not much to adapt but what you are using is designed for oils and acrylics. I have developed quite a few boxes, mostly you just need a palette and water container, and a way of holding your paper/brushes. I don't see how you can modify that particular box to work better, its going to work as is just fine, and the real issue is the overall design is based on a box from the get go.
Here is a hand held palette I set up, note your stuff does not need to be in the box to be useful, but this gives the idea of things you need. Enamel spray paint can make anything a mixing area. Just figure out a palette, brush holder, cup holder, which fits into your pochade, the canvas holder can hold a pad of paper.
The issue with what you have, is you can't really easily adjust the angle you are painting at. Gravity, the angle we paint at, that is something you want to be able to adjust constantly - hence the popularity of just using a board on a tripod, with some sort of swivel to adjust to any angle.
I used to have boxes on top, and there is a tendency for the box to tip over if the tripod can't handle the given amount of weight, either because the swivel top gets loose, or because its top heavy. I now avoid this approach entirely, just too risky. If you want to use a big box, the french easel with its own legs, or even better a table like zbukvics is a better design.
The better approach is to attach your box to the bottom of the tripod, onto the legs, then just have a board with a tripod connector on the back. And then its simpler to just have a shelf there. The board can be really light, and the box's weight actually keeps the tripod from tipping if its in strong winds since its lower.
This is also rather easy to setup yourself, a tray of some sort on the legs, then there are various ways of getting a connector onto the back of the board. The board can be anything from wood to gatorboard to coroplast ( like they use for election / real estate signs ). And because the board is on the swivel connector which adjusts position, if you got a nasty drip coming down, you can quickly change it to horizontal without knocking over your entire box.
The en plein air pro is a good example of this. It has the feature of the board having a lip, so you can use sketchbooks on it.
I have boards with 1/4" 20 tpi insert nuts on them, costs a few dollars and you just have to drill a piece of wood so it can fit into, then glue it on to light piece of wood, plastic, hardboard, gatorboard, etc. This thread size is the same as found on cameras. They also have a mounting plate you can get for about $20 on amazon. The tripod connector just screws onto this, then it attaches via a quick release system.