It's skatelite, it's about 4x harder than MDF and also water proof. It's used for skateboard ramps (my dad builds them, so was easy for him to use his scraps to do this)
There's 4 pieces of 4x2 which the hamsters don't come into contact with.
It's only 8mm skatelite, but doesn't bend at all (which is impressive if you've ever seen 8mm mdf...I have an 18mm mdf desk and it still bends in the middle, whereas this is completely rigid
)
I realise this doesn't really help since most people don't have access to it though >.< I think skatelite is a trade name and it's made the same way as MDF, but my dad said it'd be more like HDF >.<
Although I assume it's similar to some form of Masonite >.<