The Dell OptiPlex 760 (and possibly other Dells) were made before SSD's were mainstream and thus the OEM caddy for the hard disk is purpose made for the big old bulky 5.25 inch hard disk drives.

I found that a number of 5.25 inch to 2.5 inch caddy adapters wouldn't work in the OEM caddy due to the fan underneath hitting screw heads from aftermarket caddy's etc. Most of the time the SSD’s power and data ports also did not have clearance from the bracket and thus you also couldn’t plug power and data cables into them.

You can remove the hard disk fan under the OEM caddy in the OptiPlex 760 but it generates a System Fan Error on boot that can't be disabled in the bios (at least not without hot wiring the tacho yellow wire from another fan source but I didn't want to do that!)

This simple 3D printed caddy adapter (shown in red in the photos) solves it! It lifts the SSD about 10mm above the original OEM caddy base whilst being not too high the computer case lid touches in when the case is closed. The cables route perfectly and no errors!

The SSD is screwed to the 3D printed caddy adapter from underneath (I have only used two screws as shown in the photos but all four holes exist) and then clips into the original Dell OEM caddy as if it were a normal hard drive.

The measurements for the holes are per Seagate and Intel engineering drawings of hard disk drives and solid state drives respectively (common industry accepted standard on hole spacing) so other brands of SSD's will be fine.

I hope this helps someone else!