So Singer decided that the Promise should have a momentary switch to control forwards/backwards movement. I have no clue why they thought this was appropriate - it often, especially when working with tough materials like webbing, leads to me trying to do four things at once: holding down the thread, guiding the materials, holding reverse, and turning the hand-wheel.
Hence, a latch, to make the switch into a proper toggle. But a deadbolt-style latch, so I can slide the device out of the way in case I want a momentary switch.
A flexible peice of metal about 12" long (I used an old hacksaw blade)
A grinder (bench or Dremel)
A propane torch
Step 1: Make the Metal Bits
I'm assuming you'll have a different sewing machine than me, so I'm not providing measurements. It should be pretty obvious how long to make each piece.
Before separating the hacksaw blade into pieces, you'll want to grind off the teeth. Try to keep the width of the metal strip consistent, especially for the first 5 inches or so (which will become the sliding bolt of the deadbolt), by grinding off the same amount of material along its whole length. You can monitor your grinding process by watching how big the flat, shiny spots on the teeth are.
Once you've ground to depth, you'll probably need to remove the through-thickness variation - "waviness" - they introduce around the teeth so that they cut a kerf slightly wider than the rest of the blade.
Then, break the metal into peices with pliers, as shown, and grind to the profiles illustrated. You'll want to carefully deburr everything to make it smooth.
Two of the pieces, the bolt and the top restraint, need a bend in them. Heat up the area of the bend with the propane torch and then bend them 90 degrees with pliers.
I sanded most of the paint off of these pieces to make them prettier, but it wasn't really required.