For both the SSD enclosures (2in1 Thunderbolt 3 & USB Mode version and/or Thunderbolt 3 mode alone version), they all are supposed to work like that:-
there will be a silica heat dissipation gel pad be provided for every purchase
stick the gel pad onto the SSD once it is successfully installed
the gel pad should be sandwiched in between the SSD as well as the inner layer of the cover of the SSD enclosure cover
the heat generated by the SSD would then be dissipated / directed to the cover of the enclosure, and gets dissipated from there on.
the enclosure cover is the heatsink, the surface it has to direct the heat is at its max.
hence, the whole enclosure might get a little too warm to the touch when it's in operation (mind you).
***However, if you really want to max out the heat dissipation capability, you may actually look for laptop cooling pad and place it onto of it. Then whatever heat that's being directed to the surface of the enclosure would get dissipated even more efficiently. But it would not be as aesthetically pretty, though. =)
So, with the above explanation of how the heat dissipation is going to be like, there isn't rom at all for an additional heatsink there.