The taps for the M5 threads came and as I knew myself I ordered 2. And as I already knew that I am not patient I broke one at the 86 hole. I cannot get it out, but that is not such a big deal as I have plenty of those holes to mount things.
When I was done I mounted a wooden board on the steel that will be my sacrificial fixture plate.
As it is wood it should not make too much damage to the bits or the whole CNC when I make a mistake with the G-Code. And I will make mistakes.

I have used 6 M5 countersunk socket screws and I may add another 3 in the center of the x-axis, but for now, it should be fine.

The next step was to face mill the board so that I have a flat surface, at least to the geometry of the CNC gantry. I used a cheap 6mm 4 flute bit from China. It did the job.
Of course, I would like to have used something with a bigger radius, but as the shaft of the tools in an ER11 collet is limited to around 7.5mm I did not find anything better. At least for wood.

I am satisfied with how the surface came out. I started with a feed rate of 400mm/m,  and then increased it on the fly. I shot this video around 600mm/m. Later I pushed it to 1000mm/m.
I did not try more as I remembered that there was a limit listed for around 1000mm/m in the item description when I bought the CNC on eBay.

As you can see from the colour change the surface was not really flat to the ganrty geometry. Those are the different layer in the multiplex board.

When this was done I went back to my Mac to create a new g-code file in Fusion360 to mark holes in a grid with 25mm spacing.

My plan is to use screwed inserts with an M5 thread inside and a self-cutting outside thread that can be screwed into the wood. They have a hexagon socket that makes it easy to install them with a cordless drill.

I have ordered a pack of 100 pieces at eBay for a good price. They will be installed every 50mm, just like the holes in the steel plate. The holes in between will be just plain holes for simple wood dowels.

This is how far I came today. I wish those inserts would have arrived in time, then I could have finished the fixture board today. The scratch you see right next to the drill bit was from a mistake I made in the G-code. I moved the spindle a 0 z height and scratched the surface.