Here is an easy to build envelope filter using a MAX7410 switch Cap 5 pole low pass filter. You can only get low pass effects so its not as versatile as a mutron III, but you can get a nice range of effects from subtle wah to classic Jerry Garcia funk. Unlike a Mutron, you can adjust the attack which is a a very useful feature.
Layout for 1590B:
Click to access envelope-silkscreen.pdf
Click to access envelope-layout.pdf
(expresspcb file link:)