Heat sinks are essential if you want your power switch to have a long operating life. On the right are some older Pentium II heatsinks adapted to work with an SSR. This is a design adaptation that would work for smaller kilns. It would work with larger kilns with forced air from a fan. However that leaves the setup vulnerable to fan failure. Its better to go with a larger heatsink that works without a fan and if you want you can add a fan and add the the expected lifetime of the SSR part.
Its important to note that you do not just bolt an SSR down to a heatsink.
You should use some heatsink compound to make a good thermal joint between
the SSR and the heatsink.
Recommended Configuration Minimum
Heat Sink your switch on an aluminum plate 3/8" think and minimum of 6" X 6".
Alternately you can use a heatsink with fins if it has adequate mounting area.
If you use something on the smaller side you can get away with that but only
with a fan. However if the fan dies there is trouble. It is best to err on
the side of caution.