So after considering Paulvdh's comments, i came to the conclusion that the base did lack rigidity and I wasn't quite happy with the assembly. The design relied too much on proper assembly and tramming and not enough on proper machining. As i'd rather spend an hour or two extra in the workshop so i can just throw the parts together and have them aligned and square and everything. 

I've decided to add about 20 euro's of steel and an extra day of workshop time to improve the base.

The width of the base is now determined by a big chunk of precisely machined steel in the front and back instead of waterjet cut sheet steel. This will also greatly improve the rigidity of the base plate, and add some mass to the whole thing.

I've decided that this will be the final design for now, and will start making the drawings and buying the rest of the parts. The MGN rails i ordered this black friday are also in, so progress is coming.

I've uploaded a 3D pdf of the latest version. Interested people can take a look at this. Tips are still welcome and will be considered.

I'll probably be busy and away this christmas, so the next update will probably come somewhere around the half of januari. Happy new year everybody! ;)