It's really quite basic, one of those great bits of common sense improvisation.
Just an
acrylic box-frame, about 16"x20" turned upside down. That's plenty big to work in and still small enough to carry about the house and hopefully find a home on an out of the way shelf when not in use.
A few bits are added to organise your brushes and paints plus a pot of water to rinse your brushes. These need to be weighted or sufficiently stable to resist tipping over. The original plan used a plastic 'floppy-disk organiser' with lots of holes drilled in the top and a load of weighty stuff (sand?) in the bottom to hold brushes... Good luck finding one now. Just scour the Dollar/Pound shops for suitable cheap containers. The alternative is to use the useful laser-cut mdf stuff that is designed to hold paints and such. But then the project risks not being a money saver.
The last touch is a bit of padding for your wrists as the box-frame needs all its sides to remain sturdy, so you are going to be resting your hands/wrists on the edge of the frame. I used a neoprene mouse-pad.