Mount ssd on carcass of old hdd?

You can, provided that the SSD has no exposed electronics touching the metal. The "perfect" solution would be the 3.5" bracket for your SSD, though.

But there is an even easier solution - just screw the SSD into the 3.5" bay from one side. I do that all the time with desktops if they don't have dedicated SSD mounts. SSD is very light (and does not produce any vibrations), so 2 screws are more than adequate to hold it. Heck, I have seen people connect SSDs and just leave them hanging within the case between cables, that works too although it is somewhat messy for my taste ;)