If you've got room for two more knobs, you put some wobble on that echo with a PT2399 modulation daughterboard

 I designed this using the schematic from 1776 Effects Multiplex Delay modulation add-on board. I changed a few of the values but this is about 95% that design (credit where it is due). Also, I originally drew this for a fellow builder named Ben Spinks, who was actually the one to build and verify this first.

This is meant to be paired with the trails-bypass PT2399 circuit from my previous post but will work with just about any delay circuit you plug it into. However - the resistor connecting to lug 3 of the depth pot may need to be raised or lowered depending on how much wobble you want. I included the values for 1776's original design on my drawing, as well as my preferred ones.

You can place an SPST toggle between the mod board output (depth lugs 1+2) and the delay board in (usually delay lug 3 or lugs 2+3) in order to make or break the connection between the two, effectively turning the modulation on or off. The same "make or break" SPST toggle could also be placed on the 9v connection lead so that the modulation LED goes dark when the modulation is switched off. Untested, but it should work...

But this is not really necessary since the last 1/4 turn of the speed/rate pot makes the modulation so fast as to be inaudible. However, I realize some folks may want the option to "kill" it completely so I mention it for that reason.