I don’t know if this is a new idea or is there is more need other than myself however, it’s been annoying that I wasted so many R-U space due to small devices.
It would be great if a shelf such as this could be made so we don’t have devices stack up on top of each other 
I like this idea. I have a tendency to use the 1U rack shelves for the smaller switches/firewall appliances, but it would be nice to fit a standalone RAID enclosure like you’re illustrating here!
There’s this DIN Rail Kit, maybe that could be repurposed for some of this?
Adjustable Rackmount DIN Rail Kit - Rack Shelves | Server Rack Accessories | StarTech.com