Here are some Cave Swallow nests showing their variations.

This pair of nests shows the commom saucer shape,
put close to a ceiling or roof.

typical nest in a corner
This is typical for Cave Swallow nests in a corner,
again close to the ceiling.

nest under a curved roof
Here, the desire to hug the ceiling resulted in
a curved edge paralleling the curved ceiling.

nest with a lip to block light
Commonly if the light level is a bit high for Cave Swallows,
they will build a mud lip on the bright side of the nest joining
the nest rim with the ceiling.  If the light is bright on both sides,
they will build lips on both sides to keep the nest dark.  But
it will never look like a Cliff Swallow jug.

caves and cliffs coexist well
Cave and Cliff Swallows coexist well, building nests in
mutual colonies.  This Cliff Swallow nest on the right
is in a culvert, rare for Cliff Swallows.  The left nest
is a Cave Swallow nest.  Actually the Cliff Swallow
nest is a little non-typical, looking a little like a Barn
Swallow nest.  However the birds using it that day
were Cave Swallows.

swallows coexisting under a bridge
Here Cliff Swallows (left) and Cave Swallows
(right) coexist under a bridge in a predominantly
Cliff Swallow colony.