Thanks for the advice, much appreciated.
The material is Aluminium, i'm not building it but on the sketches it looks very sturdy.
Do you have any recommendations on the rubber that the nose of the drive rests on? Builder has ordered this from http://www.mcmaster.com/
UPDATE: the tubing separating the drives is 64A (says "Soft" though, which is weird, I doubt you could stretch 64A to twice its length, so I think 64A is wrong) and strip the drives rest on is 50A, I would love advice on this as we can change it.
Tubing McMaster part #: 51225K27, strip part #: 7665K18
I will hold off on the drives.
A 1TB WD Green with 1TB platters would probably be the way to go
Or a 2TB drive with higher BER (like 1e15 on the Samsung F4)
Although in saying that BER doesn't seem to make as much difference when your using mirroring in ZFS...