All of the examples I am about to share with you date to the sixth century or thereabouts. What is interesting to consider is their form on the one hand -- you can see how they have been made such so that one can step down into the pool -- but what is also of interest is the ornamentation that accompanies these particular examples. I draw your attention to this in part because as moderns we have a bad tendency to think of everything "ancient" as necessarily rustic and/or plain. Looking at these will not only help you to understand some of the symbols that were important to Christians at this time, but also realize that far from being rustic, Christians from early on placed a priority on beauty and ornamentation.
This first example is taken from the sixth century and comes from Belkata, Tunisia. One can see how spots have been carved out which would allow one to step down into the pool -- possibly even sit within it. Located at the centre of the pool, at its deepest point, is a Greek cross with the Alpha and Omega hanging from the two arms of the cross -- which comes with reference to Christ being the alpha and omega, the beginning and the end. The imagery going around the pool ranges from birds to plant life, likely with symbolic intent. Originally bordering the pool was a Latin text which now only exists in part.
Belkata, Tunisia, 6th century |
Here is another view of the same:
Kelibia, Tunisia, 6th century |
Sbeitla, Tunisia, 5th-6th century |
Of course, in ancient times, at least for larger churches, one frequently saw a building set apart as the baptistery. One can think here of the Lateran basilica's baptistery, located in the third century octagonal building behind the basilica proper. Outside of the duomo of Florence and Pisa one can also see similarly shaped baptisteries. This is certainly ideal -- but not necessarily practical or even achievable in most instances. Every church needs a place to baptize, and most won't be able to create distinct baptisteries -- and even a separate space within the church itself can be a challenge.
That said, if it it going to be done, it is certainly worth considering these paleochristian examples as sources for possible inspiration.