3.5" to 2x 2.5" hdd adapter

V4 

fix holes placement to be in par with manufacturers specs.

added more clearance for the screw head.

reduce the walls for the screw heads to be able to use standard screws.

V3 
added a bit of stiffness at the cost of a bit more of material

V2  
I've remade completely the model.
I've reduced 3.5" holes diameter to 3mm (now you are able to tap them with regular screws). The 2.5" are now a bit smaller too.

There is now a “mini” version using only 2 holes from the 3.5" side.