The problem is that the PCB is held by the front bottom 8mm diameter steel bar.
It's not efficient because nothing prevents it from turning around that round bar.
Therefore it's not possible to have it flat and leveled from the ground (well, the desk... full of dust).

So the idea is to reuse one of the setting jigs that you have used during the frame construction.

Add one of them at the front bottom of your Rapman between the black plastic ends and the diagonal ties.
Now it's going to be easier to find a design that will be able to hold the PCB leveled if we fix it on that setting jig.

Let's do some SU (to change a bit...).
That's the first prototype I ended with :

" PCB Holder V1"

It works quiet well but tend to twist the jig a bit.
So the PCB is leveled but not very firmly held at the desired angle (when you press the buttons down for ex.).

So I went back to design and ended with this improved prototype :

" PCB Holder V2"

It works really well !
Just mount 2 of them like on the following photos and your PCB won't touch the desk and suck the dust anymore.
Plus it will be much more silent.

"PCB is firmly held in place with a nice angle"

Quick tip : How to know if your fans are correctly oriented ?
If you do it "by hand" it's gonna be hard to say because you will have a cold feeling on both sides.
So just place a piece of toilet paper neer the fan to see which side is blowing and which is sucking.

You may now download that part from here.
Please leave a comment or at least a "Like it!" if you do...