I was looking for an adapter to put my 2.5" SSD drive in a 3.5" HDD bay, and found several nice designs, but none that could be screwed in from underneath, so I made one with all modern holes. It is possible to mount both the drive and the adapter in several different alignments and orientations. Does not obstruct airflow. The version with sunk screw holes can be mounted flat against a surface, with no screw heads from the small drive protruding from the bottom. The minimalist versions have lower sides, to save plastic and print time: However, it is less compliant with the 3.5" standard. Usually no biggie though.

Measurements taken from datasheet.octopart.com/FFD-25-IDEP-2048-N-A-M-Systems-datasheet-4668.pdf

This thing was made with Tinkercad. Edit online at https://tinkercad.com/things/0NNJxmj2D5o for the regular model, https://tinkercad.com/things/9QvmtPOAdhD for regular minimalist, https://tinkercad.com/things/9pyJxELLzPO for the sunk screw hole model, or https://tinkercad.com/things/e6ugwhoLcmk for minimalist sunk screw holes.

Version history: 1.0 First try 2.0 Fix all the holes so they are in the right places, made the walls higher, so the measurement from the holes center to the edge is equal in both directions and according to specs, plus some cosmetic work. 3.0 Reworked most of the model to get more precise measurements, more air-holes, a more even hex-pattern, and to be easier and more modular to rework in the future. 3.1 Adjusted hole sizes, and removed one set of legacy holes. 3.2 Fixed some minor stuff that's been bothering me, and added the version with sunk screw holes, and the minimalist versions.

Use M3 screws for the small HDD or SSD in the adapter, and 6-32 UNC 2B screws for the adapter in the 3.5" slot or drive-bay.

The adapter will fit in the drive-bay in any orientation, such as upside down, backwards and both at the same time. The drive will fit in the adapter both forward-facing and backwards, aligned to the right, left, or middle, as well as placed some ways back. If you are planning on mounting the 2.5" drive "inside" the adapter, consider printing the version with sunk screw holes

This is a fairly large print, and I suspect it would warp badly if printed in ABS.