you need to get a 2.5 to 3.5 adapter.  mount the 2.5 ssd on the adapter, then mount the adapter (as if it were a 3.5 hdd) to the chassis wall.  see service manual make sure the adapter has 6 tapped mounting holes following 3.5 hdd standard. [URL] [URL]
I bought an Inspiron 3880 with a 256GB M.2 PCIe NVMe Solid State Drive installed.  It also has a SATA connector available.  I bought a SK hynix Gold S31 1TB 3D NAND 2.5 inch SATA III internal SSD.  I don't see any place to mount it inside.  Sorry, but I don't know much about the insides of a PC.  Can anyone recommend a mounting bracket that would work, and explain how (or point me to a video) to install it in my Inspiron 3880?  Thanks in advance.
Mounting a 2.5 inch SSD in an Inspiron 3880