This is a remix of MrKindergarten's excellent letter tiles. I wanted to specialize them so that only the correct pieces would match so I added a notch to the side so that lower case "p" wouldn't fit where the lower case "d" would go.
I paused the printer at 3mm so that the base was one color and the letter a different color to further differentiate letters as well. I found that when printing in ABS, they only made a good fit when printed separately (as opposed to nestled), but that may be just for my printer.
Update 11/1/16: I found that when my son played with them, he got upset when the smaller letter would fall out ot the larger one, so I added a lip so that they are easier to assemble, as well as remove from one another.