I'd start looking at all the single-knob pedals with a switch AND just single-knob pedals, 'cause you can always throw in a mod on a switch.
The Station Wah, for example.
Could be a Bass Balls or Small Stone.
Then again, you could have a Muff with gain on a trimmer and EQ fixed with scooped/flat/bumped settings on the switch.
You could also turn that fuse-holder into a knob for a two-knobber and switch.
How 'bout a Green Ringer with Boost into Union Tube & Transistor Bumble Buzz ... ?
Really, there's so much you could do... Orange Squeezer, FV-1 one-knob reverb... Drill out the sides with some more holes for a junction box...
Maybe try for some circuit that is a tip of the hat to the enclosure's roots?