Almost all camera tripods come with a quick release mounting plate - a matchbook size piece of metal or plastic that attaches to the bottom of a camera with a screw. It slots into a receiver on the tripod head where it is secured by closing a lever.
This blog post explains how to use the same quick release mounting plate to attach a pochade box to a tripod using a t-nut, a piece of scrap wood and some wood glue. Here's a picture of a quick release mounting plate attached to the bottom of my own box.
Even though mounting plates are usually not the same across all tripod brands, they use a standard screw size - 1/4in. 20 threads per inch (you may see this abbreviated as tpi or UNC). The only exception are tripods for heavy cameras which may use a bigger standard thread.
Just as a camera has a threaded hole, you need something on the bottom of the pochade box into which the mounting plate can be screwed. The easiest solution is to use a t-nut. It's a small cylindrical nut with a flange at one end. It looks like a "T" from the side, hence the name.
Since the base of your pochade box is not thick enough to house the t-nut directly, an extra piece of wood or mdf is used as a base. This will help to even out the loads and minimize the risk of damage to your box.
The t-nut is mounted on the reverse side of this piece of wood so that the stem of the "T" faces down and the head of the "T" faces the bottom of the pochade box. The t-nut has prongs in the head which embed into the wood and prevent it from rotating. It is secured in place with a hammer.
You can buy t-nuts at any hardware store, or online. Just make sure you get the right size:
It's important to make sure that the wood is slightly thicker than the t-nut is long. You first need to make a small recess with a Forstner bit to accommodate the t-nut head, then pre-drill a suitable size hole into which the t-nut can be slotted. If you don't have a Forstner bit on hand you can scrape back a bit of wood with a chisel to create a recess for the head.
The best way to attach the piece of wood to your pochade box is to glue it with a polyurethene wood glue like Gorilla Glue. Mine has held up without any problem for two years, including numerous sessions painting on the beach in the rain. For best results follow the manufacturer's instructions for clamping and dry times, then seal the wood with polyurethene varnish once the glue is dry.
I found that mounting the wood so that the t-nut is located about 3/4 of the way towards the back edge of the pochade box gave me the best balance, but you will need to adjust that to match your own set up. Try balancing the pochade box on your fingers or on a small block of wood to find the ideal balance point (it's best to do this with the lid up and holding a panel, just as if you were painting). Placing the nut in the right spot will minimize any twisting of the tripod head when it's in use.
I used a ball mount head on a medium-duty carbon fiber tripod which is rated for 20lbs. I think this is the best choice of tripod for a plein air painter and it's more than strong enough for my pochade box, even with its glass palette. I'd recommend against buying a top-of-the-line tripod—it will just end up getting covered in paint, and a painter's tripod tends to get handled very roughly.
Here's a close-up of my home-made pochade box mounted on the tripod on location at Lake Union in Seattle, WA.
Set up at Lake Union Park in Seattle, WA.