Well, that was a productive weekend! The second quad, is almost ready to fly.
I decided to try something I saw somewhere in a forum, and that is mounting the motors not on top of the arms, but below them. The reasoning behind this is that a propeller can suck air from anywhere it wants, but it has to blow the air out at the underside. The sucking of air is not really diminished by the presence of obstacles, whereas the same presence in the air that it blows has quite a big effect, since this airstream is highly directional. So, propellers at the underside it is going to be. This of course makes the presence of a landing gear quite important, and I have still to figure out a way, or more accurate, a place to mount them to the airframe.
Anyway, the quad has been under its own power, and I was able to start all engines via the transmitter, so all looks promising. Next on the todo-list: Route all the wires which go to the ESC’s (and via those to the engines) and connect all the necessary wires to the receiver (and make it all look tidy). Also the landing gear needs some looking into, as well as something to mount the battery to.