Duck tape... Lol what I did for a while was just kinda hold it up with some long craft sticks. Kinda like giant tooth picks. I had them just laying around the house so I stuck the through the screw holes in the HDD rack and laid the ssd on them. As long as you don't move the PC something like that should work
If you have any motherboard mounts (normally gold but sometimes silver) laying about then try screwing these into the screw holes on the side of the SSD. If you do that with any luck you should be able to mount the SSD in a standard 3.5 HDD Bay. Insert the screws in the mounts which have now spaced your SSD to 3.5. See how you go.
The reason I like them is that the adapters have built in flexibility for mounting and they come with a small bag of mounting screws. In addition, the drive bay adapters are a little bit shorter than the full length adapters. The slightly shorter length avoids possible interference with drive bay covers on the front of the case, especially Lian Li cases.
An SSD has no moving parts, so it isn't as necessary to mount it properly like a HDD (which you don't want shaking/vibrating). I've had an SSD in my system for almost 2 yrs and it just sits on the bottom of my 5.25" bay with no mount or screws.
Now, if you want your case to look nice and neat, it is worth investing in a 2.5" to 3.5" adapter to mount it properly.