I find it useful to start pretty much any DIY project by creating a simple actual size 3d model with https://tinkercad.com. This allows me to get the idea of how the thing will look like in the end and make sure, that everything fits well together before I order missing components.

For those, who are familiar with Pocket Operators - I use audio/sync output from the first PO and stereo output from the second and the third. This way I can switch between two modes: external sync and standalone mode, where sync is being produced by the first PO. I use external sync with my Korg Volca Suitcase, which I will hopefully cover in another instructable at some day.

Sync mode settings for POs:

External: SY3 -> SY2 -> SY2

Standalone: SY1 -> SY2 -> SY2