Here is a tips for mounting encoder : Cheap and simple !
2 littles pins from cutted resistors soldered with the “legs” of the encoders wich doesn’t fit in the breadboard holes …
Solder the pins, folded with the encoder pushed on the breadboard, on the bottom side:
And you’re good for tweaking knobs all nightlong !
Here is my SID step C design …
Now I have to think more on the front panel ;) ;D
Just an alternative suggestion, if you hold a pair of pliers perpendicular to the angle of the metal on the flat pin, and try to squish it flat the other way, they curl around and will usually slide right on in just like a normal pin…
Basically you fold it in half 
Trying to beat my designs  ;) Looks great so far!!
/edit: da spelling…
;D
i need portable small device … so i try to compact the SID step C … ;D
