All the hidden tracks have been laid and tested, so it's time to cover them and loose the comfort that gives you the possibility to work with the frame opened.
I tried to find a way to do it to be able to dismount it in case of "emergency". And the only possibility was to screw the base instead of gluing it. And it's been a good idea because it's faster and stronger. There is also the advantage to be able to level it. For sure it will not be easy to dismount if some day it's needed, but at least there is the possibility without breaking a lot of things.
The first thing to do is to be far-sighted and lower some notches on the top of the ribs for passing wires. Make a big hole if you preview access from the side can be needed (big enough for your hand at least). It's still difficult to know how many wires and where will they go through, so my recommendation is to drill four notches in each rib crossing like in the image:
In order to fix the base board I used metallic squares screwed to the ribs. In the base board I drilled a hole for each square and countersink it. The inox screws I'm using have conical head so once in the hole they are leveled.
These are the tools and material used: