Polystyrene is easy to work with.
You can drill or saw the material quite easily.
It is also possible to cut it with a knife.
At the same time the material is solid enough to give you more mounting options.
The material can also be used to encase your electronics.
The sheets MakerBeam sells are 300mm x 200mm x 3mm.
The 3mm thickness will slot right into the MakerBeam.