Before I start cutting up some steel, I was wondering if such a thing is made.

I have my 3 hard drives in their bays and even though the machine is not always in 24/7 I feel that they are getting too hot.

There is plenty of room so that I could hang down the middle hd below the bays which would give more airflow around these.

So do they make a simple pressed tin part.

I have looked in companies like dabs and maplins (UK) without success.

I think people dont think about the fluff in their machines.

I have taken to regularly blowing out the fans as the build up is astonishing (also considering I have no carpet).

try velcro or zip strips to hang it.

I would hope he wants something a little more sturdy and solid than zip ties and velcro.
As far as premade stuff I'm not sure, but it would be pretty easy to fabricate, you could probably just use some aluminum flashing used for duct work and cut it and bend it on your own.

Aluminum... A drill... Some screws.. Viola!

Thanks folks, that was kinda the plan, I just thought it would be a standard accessory. Guess most cases are too small though.

He could use some duct tape as reinforcement

Mmmmmmm duct tape that will get nice and sticky lol

Duct tape will get to hot. Then a bunch of gluey stuff will get all over everywhere.

How do I know this? Sadly from experience.

Coat hangar would probably be a better solution then