I had a bunch of 2.5" SAS drives that I wanted to fit into 3.5" drive caddys. While I could screw 2 screws in and it would work, I'd feel a little better about having a bracket in there to hold the drive a little more secure. I still need to put the screw holes in to secure it to the 3.5" caddy and the 2.5" drive- I will edit this thing once I print out a test. I designed it based on measurements of a production part that serves this same purpose. This was for a Dell server, but should work for any 3.5" drive caddys.

edit 6/13/19:
Turns out this insert fits perfectly into the drive caddy (first try, too!). There is really no need to add screw holes, since the drives are secured to the caddy on one side and the insert squeezes perfectly in and holds the other side with friction. I left a lot of empty space and added holes to the front for proper ventilation- in fact, it has more vent holes that the production piece that serves the same purpose. I would be willing to add screw holes if anyone is interested in them, but I am using it as it is.