Ok, I've made some changes, thanks for helping me!
This is the result, don't know if it's really better:
As you can see I've used some bigger rocks at the base of the wood, and some smaller ones on the right side (there just there for a hard barrier between the sand and the aquasoil.
You have to bear in mind that the stones on the right will be buried almost completely.
I think I payed about 40 euros for that piece (about 50 dollars).
substrate will be ada nile sand and aquasoil amazonia
The tank is 130 litres (34g) and measures 80x40x40 (31x16x16 inches).
The fixture is a 'pimped' Arcadia overtank luminaire; I took the 2x25w t8 lights out, I made a new reflector, and replaced them with 2x55w PC's (dimmable).
Also, normally, OTL's come with mounting brackets, but I think they look nicer, hanging from the celing.
I think he achieved this with multiple smaller pieces of wood.
The thing is, I only have 16 inches to work with so I can't move the wood any more to the back (due to them st*pid support brackets).