The key to making great ambient music appears to be drenching the sound in reverb. Based on this, reverb ranked highly for effects modules that I want to build.  The design I settled on is Music Thing Modulars Reverb, although I omitted the analog spring reverb circuit and simply built the digital circuit.

The circuit is based on the Accutronics Digi-Log Reverb brick, a digital reverb circuit based on three PT2399 delay chips. The module uses a vactrol to mix the wet and dry signals, has adjustable feedback, and a subtle amount of EQ control to emphasize either the low or high end of the sound.