In my experience those kind of 3.5" hard drive caddies are meant to mount the 3.5" hard drive from the bottom. It looks like your adapter rail has holes on the bottom, at least the one side does. Have you tried to see if you can mount the rail adapter to the disk caddy from the bottom instead of the sides? Are there enough holes and do they line up? If so, you could mount the adapter rails to the caddy from the bottom, then mount the 2.5" SSD to the adapter from the sides like you have it.