As mentioned, one of the multiplex sheets was too thick (9 instead of 8 mm due to imperial to metric conversion). Therefore, the parts from that sheet didn’t fit in their cut-outs.. and since I didn’t want to sand them down completely, I took out the route jig for my Dremel:

Came out neat didn’t it?

And now, it fits like a glove, including the recess for the balsa cross-beam right next to it:

And next to the tabs, I wanted to alter something else too. My wing tube sleeves are 42,5 mm in diameter and my wing tube 40 mm. The box for the wing tube in the kit is also 40 mm, which would be perfect to accept my wing tube, but I rather have the sleeve to go through the fuselage as well. So I made a little CAD drawing to see how big the slots needed to be to accept the sleeve. Added benefit, I can use those pockets to fill them with epoxy making the connection even stronger:

First, mark the slots with a pencil:

And then route them free-hand:

And on the backside of those parts, the tabs needed thinning as well:

And in the end, the wing tube box slides together perfectly well:

This is how it looks from the inside:

And the wing tube sleeve fits in very nicely:

By the way, don’t know if I already mentioned it, but the wing tubes are Strongal tubes from Petraush in Germany, awesome stuff!