The first level of the rack has regular feet : vertical beams attached to the main surface.
But for the second layer, i need to be able to secure those feet to the first one. To do this, i decided to simply add a cross beam at the bottom of the foot assembly : instead of havint an inverted U shape, it is now a square shape (with a square hole in the middle):
As you can see, i kept the general shape of the previous layer, but added this bottom cross beam.
It will allow me to drill it, and secure it to the bottom layer with screws, thus making it really stiff.
You can see the two beams that will help me make the surface of this level, and obviously, i need to add 3 perpendicular beams (one on the top of each foot assembly) to have everything at the same level.
But for that, i secured the feet to the beams with screws, to have it beeing in the right position :
At this point, i measured the distance between the beams, and cut pieces to fill this part.
I turned it over to screw those beams from below :
On the picture above, you can see the second level from below. As it's built, it doesn't stay square by default, as only one long screw secures each corner of the rectangle, so it could easily end up not beeing square.
To correct that, i added metal L brackets at the junction of the long beams and each foot. I painted them brown, but perhaps a lighter brown would have been better.
This not only keep the top square on the X/Y plane, but helps keeping the feet perpendicular on the Z axis.
It's not on the picrue below, but the cross beams are secured with 2 5x70mm wood screws from below.
Next step is carving a groove in the top of the wood to be able to drop a 15mm thick plywood plank in each of the large rectangles, as for the first level; then rounding the edges with the router and thus oiling the wood.