wood frame, load bearing calculation
How long are the 6 x 12cm beams?  

With just two sides and a top you are going to have problems with a freestanding unit wanting to parallelogram unless you put some corner bracing in the upper corners that should span down at least 25 to 35% of the height. This of course can be mitigated by fastening into the existing building structure. 

It is my feeling that the sides being framed from 2.4 x 4.8cm members is pretty flimsy. I would use something that is much sturdier more like on the order of 4.8 x 9.6cm for the side members.