@rpardee
That case design is a horror show. Standard practice for Dell, though.
I don't see any good place to mount a 3.5 in HDD.
If I were you, I would park that drive right on top of the power supply, with these under the 4 corners to reduce vibration and to keep it from scooting around.
https://www.amazon.com/Moongel-RTOM-Resonance-Pads/dp/B00GXNZP80/
@rpardee
Also, putting heavy things on top of a laptop is risking some very expensive damage.
@rpardee
It will actually be superior to hard-mounting the drive anyway, as long as this system is staying in a fixed location.
Hot glue it to the PSU enclosure.
@FreedomATX
Uh, I hope you're kidding.
Depending on where the glue goes on the enclosure, I'd give it as high as 50/50 that the glue stays solid during use!
@voltronic ah, bummer. I do have one USB-C port, tho it's on the front of the machine, so less desirable.
I'll do a little more research to see if I can find something to actually screw the drive into & if not will go w/your idea of perching it on the PSU.