I have cooked up a compressor design that uses 3 2N7000 NMOS FETS and one op amp. Its very easy to build and uses a LED as a peak detector and compression level indicator, as it lights up with varying intensity proportional to compression level.
Using a MOSFET as a voltage controlled resistor requires very small signal levels or it will cause distortion. To solve this, I employed negative shunt feedback to buck the input signal at the input node. This results in signals in the range of ten mV across the voltage controlled resistor.
LINK to Schematic: