Once, someone wanted to help us with SATA for OpenRex, He has never finish it, but it can give you an idea how to do it. Here are the files:
https://github.com/hemalchevli/openrex-hdd-add-onPS: You may not be able to mount anything under CPU as there will be probably components (unless you find a stand off solution - but still as @Paul van Avesaath mentioned, you need to be careful about heat dissipation).
PSS: There are also different solutions, e.g. have a look here:
https://www.imx6rex.com/2017/09/sata...or-rex-boards/3D PDF is in attachment, here is screenshot from the 3D pdf: