I found relatively cheap HDD caddys for the DVD slot in a CF31.
But they say, I need the original DVD drive, and should use the frame from this drive

Why? Licence?
I do not have a DVD drive. If i would buy a used DVD drive (local ebay ~$82

I would use this caddy not for a "second" drive, I would usa a 840 Pro SSD from Samsung as System drive.
And the regular HDD caddy, the big box with foam, for a 5TB 2.5" HDD from Seagate.
I can not install the system from this DVD drive, because I had to remove the SSD..., The same with installing software from CD oder DVD.
And buy a drive just for the frame, is insane.
Are there offers for frame only? If this is only a thin plastic frame, without its own connector, some people could print it with 3D printers, and sell it on ebay for some dollars...
Do HDD caddies also fit without frames?
Will they then be "shaky"?
Thanks...