What Javi said. When it's a tight fit I also solder pots outside of the box, only I use the box turned upside down (screw the pots in with the terminals on the outside, no cardboard necessary) and when done just turn the box over and stuff it in. Mind the jacks, though. When the box is on the inside, left/right are reversed.
No, I never made that mistake...
Naturally, this works perfectly with symmetrical boxes, once you start putting pots and footswitches all over the place, cardboard may become necessary.