The MD1000 isn't the only DAS chasis out there. There are cheaper ones that don't carry a Dell badge. I think if you want to put your own hardware in then 1U is going to be easier to source.
There are a lot more short 1U and 1U empty chassis out there than there are bigger ones. Mostly because if you're buying something that big, it's rare you'd not want it to ship ready to use.
I'm not sure how cheap you think you can do this though, £500 for a 16bay chassis with SAS expander is cheap. A 4 bay buffalo drive station USB/eSATA would set you back £300....