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.

Schematic Here:

Layout for 1590B:

Click to access envelope-silkscreen.pdf

Click to access envelope-layout.pdf

(expresspcb file link:)

http://www.fileswap.com/dl/IOjVRKVnuI/

Audio Sample: