Without getting into specifics, BLS opens the discussion by suggesting traditional symbolism for the design of the font:
§ 68 § Water is the key symbol of baptism and the focal point of the font. In this water believers die to sin and are reborn to new life in Christ. In designing the font and the iconography in the baptismal area, the parish will want to consider the traditional symbolism that has been the inspiration for the font’s design throughout history. The font is a symbol of both tomb and womb; its power is the power of the triumphant cross; and baptism sets the Christian on the path to the life that will never end, the “eighth day” of eternity where Christ’s reign of peace and justice is celebrated.
Six sides (tomb, Friday) or eight sides (womb, Sunday)? Stone and suggestive of a mausoleum? Colors in tile or frescoes?
Iconography–that’s the term used. In my parish, we have the ambry set nearby, and a cross inlaid into the bottom of the font.
My own recollection of baptism, which was conducted in a baptistry, is that behind the priest, I saw a stained glass piece that contained the Apostle’s Creed. A helpful “crib sheet,” but I tried to focus on the water instead. Something less literal would be great. What do you think of this banner over the cathedral font in St Mary’s, Winnipeg?
All texts from Built of Living Stones are copyright © 2000, United States Conference of Catholic Bishops, Inc. All rights reserved. Used with permission.
Catholic Sensibility 
The chief font in Catholicism – the font in the baptistery of the Lateran archbasilica – is a curious model of tublike sarcophagus. Here’s a picture (without the elaborate font cover that kinda makes it look more like a sarcophagus):
And here’s a shot with the font cover: