I was also thinking about getting a 30cm long and 1-2 mm thick metal or aluminium plate.

As the HD cage will be removed, you can mount that plate by making a 90° bow at one end of the plate (approx. 3cm wide), drill some holes in the bowed part, and mount it in the provided holes from the HD cage.

And while you're at it, you can make some fanholes in that plate, so you can make a push-pull config.

I hope you understand what I mean.