I was gifted a 10TB external hard drive a little while ago and I was curious to know if it is possible to mount the external drive on to the top of my tower.
I have a lot of screw holes on the top so it should not be an issue making a bracket of some sort to the top of the case.
I'm guessing you want to mount the external to the chassis so that it travels wherever the PC does? If so and depending on the drive, you may be able to shuck the internal drive and install it inside your machine, assuming you have a free slot open and spare SATA cables.
If you are open to this idea, I would suggest you hit YouTube and see if someone has made a tutorial for your drive. I've not seen any 3.5" external that use custom PCBs, but be aware WD is known for it on some of its lineup. That said, the MyBook and Easystore lineup is also well known to use WD Red drives, just know that once you open the unit, your warranty is gone.
Outside of this, keeping your original idea intact, I would look for some short sheet metal screws with a self tapping point. Keep in mind you will still need an external power source for this drive.