There is an Osprey book (by the way, translated into Spanish with the title "América Latina: de la conquista a la independencia") where you can find some shields' designs. In general the representations you can find are rather geometrical, with vivid colours.
Here you have some representations. The first one is for Aztecs, the second one for Mayans:
It seems that close combat fighters used a round shiled whereas square ones were used more by slingers. At least that is how they are represented in the Osprey book. The problem is that I have seen some Mayan and Aztec paintings and it seems that it is not easy to figure out if that is a round or a square shield or a square shield with the edges rounded. According to these gliphons, the Mayans could have perfectly used the square version too. What I have not seen is any representation of Aztects with a square shield. More information here:
http://www.realalternativesite.com/maya ... -1302.html
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