Following up to my own post, a tip for pre-molding the ice cream.

If using a 9-inch cake layer, line an 8-inch cake pan with plastic wrap with
the edges laying over the edge of pan. Spoon and press ice cream into pan to
fill and level off flat. Cover with another piece of plastic wrap and return
to freezer until frozen hard.

The plastic wrap makes it easy to remove the ice cream from the pan, and the
size is perfect for placing on the cake layer.

Obviously, depending on the size and shape of cake layer you use, a slightly
smaller pan of similar shape can be used for the ice cream.

Wayne