The heat transfer plates between cells (conducting heat to the plate with liquid circulating in it) in pouch cell modules is normally thin aluminum, and covers the width of the cell from the plate, so typically twice the height of an 18650 cell.
Your call, and I understand the issue with joining the fin to the base plate, but without curving around the cell contact between the cooling fin and the cell case will only be a line - no area. Between the fin and the rest of the cell case is whatever you use for potting, but as I noted before...
The Tesla cooling snake only follows the cell wall for less than one-sixth of the cell circumference, but that's a lot more than line contact. That plates (fins) are a natural match for pouch cells, but not for cylindrical cells.
if you see the Tesla snake cooling line it is an eye opener. I can't believe there is much water flow at all, in addition it is wrapped in a thick ribbed rubber jacket. So whether it touches 1/6 or less I have doubts it is significant at all. I think the potting is doing the lions share.