After reading a ton of FOG posts I finally start with the cart project.
Here a dry fit of the torsion box. The strips are 15mm (~1/2") thick by 80mm (~3") high with half lap joint. I Made the "groves" using a dado blade on the table saw. The worst part was to get a very snug fit for the half lap joint. On the picture you can see that the strips have to be cut in length. I put the grid together to make sure everything was perfect before I remove the dado blade
![Big Grin [big grin]](http://festoolownersgroup.com/Smileys/default/biggrin.gif)

and here after the cuts. Used the Kapex to have all the strips the same length on both axis. I am very happy with the result. I will add a frame 18mm strip around the grid to close it.

On that picture you can see more closely how snug are the joints. They seems to be not flush but it's because the ply used to sit the grid is not leveled yet. That's the next part I have to work on before glue up.

Now on the assembly? My plan is to put together the frame and then square it on a flat surface. Once it's perfectly flat and square add the grids. The grid will be sandwiched between two BB ply, not sure yet if it's going to be 12mm, 15mm or 18mm.
Any advice and do I need to glue up the grid strips to keep from warping?
That surface will be my table saw outfeed table and assembly table.
Plan A: Make a custom MFT holes pattern to fit between the grid. The grid is 200mm (~7 3/4") square.
Plan B: Add a sliding MFT top that will sit on the top of the torsion box raising the working surface some 100mm (~4")