This is the modified EA Tremolo circuit as seen on runoffgroove. I believe it’s the same circuit as the BYOC Tremolo.
I decided to lay this one out with the option of using board-mounted pots. If you want to do it this way get the 90 degree 16mm Alpha PCB pots for the “Depth” and “Rate” pots, and get a regular 16mm Alpha solder lug pot for the “Volume”. This should fit nicely in a 1590B. Solder the PCB pots underneath the circuit board (isolate the pot housing with some electrician’s tape or something similar). You’ll need to drill holes 30.48mm / 1.2″ apart for these. And the “Volume” pot can go wherever you feel like it, mounting the board on the two PCB pots should leave enough room underneath there that the volume pot can fit (isolated housing, of course).
Update (18.07.2012): Corrected the pot labeling; there were two depth pots and no rate pot.
Update (07.08.2012): Thanks for verifying the layout, Sam.