Post the schematic?
I guess I can dig it up myself..
According to the non-official (confirmed?
) schematic I found here: https://forum.bassbuzz.com/t/boss-odb-3 ... -mod/13162
The gain control is a dual gang, but wired as two rheostats.
I bring this up because when we're talking about racks, my mind goes directly to two channel, which is nice in stereo (shared controls).
So, some work around for the gain section may benefit, unless dual mono controls is your jam.
I would also suggest fanning the tone controls out into their own discrete knobs instead of going concentric.
From there the scope of work is as simple as placing two circuits into the large rack case and hooking them up. I would think that going four channel might best be realized with a pair of stereo controls on top of one another with provision to route stereo pair A into stereo pair B circuitry in either a parallel or series manner - actuated either by front panel switch or facilitated by not plugging into those jacks (so A normals into B unless interrupted). Series/parallel can be realized with some relays, solid state switching, or physical multi throw switches (though these can be problematic to source for a variety of reasons).
I think the biggest hurdle is getting the gain section squared away, and that could be done with LDR given it's only a pair of rheostats per channel.