Use some duck tape. The SSD weigh next to nothing. Else use some velcro. There are so many ways to get a SSD to stay put. Otherwise drill some holes 2 screws and the SSD will be put in place nicely. It all comes down to how you want it all to look when it is all said and done.
See how I drilled 6 extra holes in the space/bracket from the 3.5" drive and that let me mount 2 SSD's in its place:
https://i.imgur.com/XRACaRT.jpg
It is the opportunity to make the case really yours and customize to your liking.